Method and apparatus for high rate packet data transmission

ABSTRACT

In a data communication system capable of variable rate transmission, high rate packet data transmission improves utilization of the forward link and decreases the transmission delay. Data transmission on the forward link is time multiplexed and the base station transmits at the highest data rate supported by the forward link at each time slot to one mobile station. The data rate is determined by the largest C/I measurement of the forward link signals as measured at the mobile station. Upon determination of a data packet received in error, the mobile station transmits a NACK message back to the base station. The NACK message results in retransmission of the data packet received in error. The data packets can be transmitted out of sequence by the use of sequence number to identify each data unit within the data packets.

BACKGROUND OF THE INVENTION

[0001] I. Field of the Invention

[0002] The present invention relates to data communication. Moreparticularly, the present invention relates to a novel and improvedmethod and apparatus for high rate packet data transmission.

[0003] II. Description of the Related Art

[0004] A modern day communication system is required to support avariety of applications. One such communication system is a codedivision multiple access (CDMA) system which conforms to the“TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard forDual-Mode Wideband Spread Spectrum Cellular System”, hereinafterreferred to as the IS-95 standard. The CDMA system allows for voice anddata communications between users over a terrestrial link. The use ofCDMA techniques in a multiple access communication system is disclosedin U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESSCOMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S.Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMSIN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee ofthe present invention and incorporated by reference herein.

[0005] In this specification, base station refers to the hardware withwhich the mobile stations communicate. Cell refers to the hardware orthe geographic coverage area, depending on the context in which the termis used. A sector is a partition of a cell. Because a sector of a CDMAsystem has the attributes of a cell, the teachings described in terms ofcells are readily extended to sectors.

[0006] In the CDMA system, communications between users are conductedthrough one or more base stations. A first user on one mobile stationcommunicates to a second user on a second mobile station by transmittingdata on the reverse link to a base station. The base station receivesthe data and can route the data to another base station. The data istransmitted on the forward link of the same base station, or a secondbase station, to the second mobile station. The forward link refers totransmission from the base station to a mobile station and the reverselink refers to transmission from the mobile station to a base station.In IS-95 systems, the forward link and the reverse link are allocatedseparate frequencies.

[0007] The mobile station communicates with at least one base stationduring a communication. CDMA mobile stations are capable ofcommunicating with multiple base stations simultaneously during softhandoff. Soft handoff is the process of establishing a link with a newbase station before breaking the link with the previous base station.Soft handoff minimizes the probability of dropped calls. The method andsystem for providing a communication with a mobile station through morethan one base station during the soft handoff process are disclosed inU.S. Pat. No. 5,267,261, entitled “MOBILE ASSISTED SOFT HANDOFF IN ACDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the presentinvention and incorporated by reference herein. Softer handoff is theprocess whereby the communication occurs over multiple sectors which areserviced by the same base station. The process of softer handoff isdescribed in detail in copending U.S. patent application Ser. No.08/763,498, entitled “METHOD AND APPARATUS FOR PERFORMING HANDOFFBETWEEN SECTORS OF A COMMON BASE STATION”, filed Dec. 11, 1996, assignedto the assignee of the present invention and incorporated by referenceherein

[0008] Given the growing demand for wireless data applications, the needfor very efficient wireless data communication systems has becomeincreasingly significant. The IS-95 standard is capable of transmittingtraffic data and voice data over the forward and reverse links. A methodfor transmitting traffic data in code channel frames of fixed size isdescribed in detail in U.S. Pat. No. 5,504,773, entitled “METHOD ANDAPPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to theassignee of the present invention and incorporated by reference herein.In accordance with the IS-95 standard, the traffic data or voice data ispartitioned into code channel frames which are 20 msec wide with datarates as high as 14.4 Kbps.

[0009] A significant difference between voice services and data servicesis the fact that the former imposes stringent and fixed delayrequirements. Typically, the overall one-way delay of speech frames mustbe less than 100 msec. In contrast, the data delay can become a variableparameter used to optimize the efficiency of the data communicationsystem. Specifically, more efficient error correcting coding techniqueswhich require significantly larger delays than those that can betolerated by voice services can be utilized. An exemplary efficientcoding scheme for data is disclosed in U.S. patent application Ser. No.08/743,688, entitled “SOFT DECISION OUTPUT DECODER FOR DECODINGCONVOLUTIONALLY ENCODED CODEWORDS”, filed Nov. 6, 1996, assigned to theassignee of the present invention and incorporated by reference herein.

[0010] Another significant difference between voice services and dataservices is that the former requires a fixed and common grade of service(GOS) for all users. Typically, for digital systems providing voiceservices, this translates into a fixed and equal transmission rate forall users and a maximum tolerable value for the error rates of thespeech frames. In contrast, for data services, the GOS can be differentfrom user to user and can be a parameter optimized to increase theoverall efficiency of the data communication system. The GOS of a datacommunication system is typically defined as the total delay incurred inthe transfer of a predetermined amount of data, hereinafter referred toas a data packet.

[0011] Yet another significant difference between voice services anddata services is that the former requires a reliable communication linkwhich, in the exemplary CDMA communication system, is provided by softhandoff. Soft handoff results in redundant transmissions from two ormore base stations to improve reliability. However, this additionalreliability is not required for data transmission because the datapackets received in error can be retransmitted. For data services, thetransmit power used to support soft handoff can be more efficiently usedfor transmitting additional data.

[0012] The parameters which measure the quality and effectiveness of adata communication system are the transmission delay required totransfer a data packet and the average throughput rate of the system.Transmission delay does not have the same impact in data communicationas it does for voice communication, but it is an important metric formeasuring the quality of the data communication system. The averagethroughput rate is a measure of the efficiency of the data transmissioncapability of the communication system.

[0013] It is well known that in cellular systems thesignal-to-noise-and-interface ratio C/I of any given user is a functionof the location of the user within the coverage area. In order tomaintain a given level of service, TDMA and FDMA systems resort tofrequency reuse techniques, i.e. not all frequency channels and/or timeslots are used in each base station. In a CDMA system, the samefrequency allocation is reused in every cell of the system, therebyimproving the overall efficiency. The C/I that any given user's mobilestation achieves determines the information rate that can be supportedfor this particular link from the base station to the user's mobilestation. Given the specific modulation and error correction method usedfor the transmission, which the present invention seek to optimize fordata transmissions, a given level of performance is achieved at acorresponding level of C/I. For idealized cellular system with hexagonalcell layouts and utilizing a common frequency in every cell, thedistribution of C/I achieved within the idealized cells can becalculated.

[0014] The C/I achieved by any given user is a function of the pathloss, which for terrestrial cellular systems increases as r³ to r⁵,where r is the distance to the radiating source. Furthermore, the pathloss is subject to random variations due to man-made or naturalobstructions within the path of the radio wave. These random variationsare typically modeled as a lognormal shadowing random process with astandard deviation of 8 dB. The resulting C/I distribution achieved foran ideal hexagonal cellular layout with omni-directional base stationantennas, r⁴ propagation law, and shadowing process with 8 dB standarddeviation is shown in FIG. 10.

[0015] The obtained C/I distribution can only be achieved if, at anyinstant in time and at any location, the mobile station is served by thebest base station which is defined as that achieving the largest C/Ivalue, regardless of the physical distance to each base station. Becauseof the random nature of the path loss as described above, the signalwith the largest C/I value can be one which is other than the minimumphysical distance from the mobile station. In contrast, if a mobilestation was to communicate only via the base station of minimumdistance, the C/I can be substantially degraded. It is thereforebeneficial for mobile stations to communicate to and from the bestserving base station at all times, thereby achieving the optimum C/Ivalue. It can also be observed that the range of values of the achievedC/I, in the above idealized model and as shown in FIG. 10, is such thatthe difference between the highest and lowest value can be as large as10,000. In practical implementation the range is typically limited toapproximately 1:100 or 20 dB. It is therefore possible for a CDMA basestation to serve mobile stations with information bit rates that canvary by as much as a factor of 100, since the following relationshipholds: $\begin{matrix}{{R_{b} = {W\frac{\left( {C/I} \right)}{\left( {E_{b}/I_{o}} \right)}}},} & (1)\end{matrix}$

[0016] where R_(b) represents the information rate to a particularmobile station, W is the total bandwidth occupied by the spread spectrumsignal, and E_(b)/I_(o) is the energy per bit over interference densityrequired to achieve a given level of performance. For instance, if thespread spectrum signal occupies a bandwidth W of 1.2288 MHz and reliablecommunication requires an average E_(b)/I_(o) equal to 3 dB, then amobile station which achieves a C/I value of 3 dB to the best basestation can communicate at a data rate as high as 1.2288 Mbps. On theother hand, if a mobile station is subject to substantial interferencefrom adjacent base stations and can only achieve a C/I of −7 dB,reliable communication can not be supported at a rate greater than122.88 Kbps. A communication system designed to optimize the averagethroughput will therefore attempts to serve each remote user from thebest serving base station and at the highest data rate R_(b) which theremote user can reliably support. The data communication system of thepresent invention exploits the characteristic cited above and optimizesthe data throughput from the CDMA base stations to the mobile stations.

SUMMARY OF THE INVENTION

[0017] The present invention is a novel and improved method andapparatus for high rate packet data transmission in a CDMA system. Thepresent invention improves the efficiency of a CDMA system by providingfor means for transmitting data on the forward and reverse links. Eachmobile station communicates with one or more base stations and monitorsthe control channels for the duration of the communication with the basestations. The control channels can be used by the base stations totransmit small amounts of data, paging messages addressed to a specificmobile station, and broadcast messages to all mobile stations. Thepaging message informs the mobile station that the base station has alarge amount of data to transmit to the mobile station.

[0018] It is an object of the present invention to improve utilizationof the forward and reverse link capacity in the data communicationsystem. Upon receipt of the paging messages from one or more basestations, the mobile station measures thesignal-to-noise-and-interference ratio (C/I) of the forward link signals(e.g. the forward link pilot signals) at every time slots and selectsthe best base station using a set of parameters which can comprise thepresent and previous C/I measurements. In the exemplary embodiment, atevery time slot, the mobile station transmits to the selected basestation on a dedicated data request (DRC) channel a request fortransmission at the highest data rate which the measured C/I canreliably support. The selected base station transmits data, in datapackets, at a data rate not exceeding the data rate received from themobile station on the DRC channel. By transmitting from the best basestation at every time slot, improved throughput and transmission delayare achieved.

[0019] It is another object of the present invention to improveperformance by transmitting from the selected base station at the peaktransmit power for the duration of one or more time slots to a mobilestation at the data rate requested by the mobile station. In theexemplary CDMA communication system, the base stations operate at apredetermined back-off (e.g. 3 dB) from the available transmit power toaccount for variations in usage. Thus, the average transmit power ishalf of the peak power. However, in the present invention, since highspeed data transmissions are scheduled and power is typically not shared(e.g., among transmissions), it is not necessary to back-off from theavailable peak transmit power.

[0020] It is yet another object of the present invention to enhanceefficiency by allowing the base stations to transmit data packets toeach mobile station for a variable number of time slots. The ability totransmit from different base stations from time slot to time slot allowsthe data communication system of the present invention to quickly adoptto changes in the operating environment. In addition, the ability totransmit a data packet over non-contiguous time slots is possible in thepresent invention because of the use of sequence number to identify thedata units within a data packet.

[0021] It is yet another object of the present invention to increaseflexibility by forwarding the data packets addressed to a specificmobile station from a central controller to all base stations which aremembers of the active set of the mobile station. In the presentinvention, data transmission can occur from any base station in theactive set of the mobile station at each time slot. Since each basestation comprises a queue which contains the data to be transmitted tothe mobile station, efficient forward link transmission can occur withminimal processing delay.

[0022] It is yet another object of the present invention to provide aretransmission mechanism for data units received in error. In theexemplary embodiment, each data packet comprises a predetermined numberof data units, with each data unit identified by a sequence number. Uponincorrect reception of one or more data units, the mobile station sendsa negative acknowledgment (NACK) on the reverse link data channelindicating the sequence numbers of the missing data units forretransmission from the base station. The base station receives the NACKmessage and can retransmit the data units received in error.

[0023] It is yet another object of the present invention for the mobilestation to select the best base station candidates for communicationbased on the procedure described in U.S. patent application Ser. No.08/790,497, entitled “METHOD AND APPARATUS FOR PERFORMING SOFT HANDOFFIN A WIRELESS COMMUNICATION SYSTEM”, filed Jan. 29, 1997, assigned tothe assignee of the present invention and incorporated by referenceherein. In the exemplary embodiment, the base station can be added tothe active set of the mobile station if the received pilot signal isabove a predetermined add threshold and dropped from the active set ifthe pilot signal is below a predetermined drop threshold. In thealternative embodiment, the base station can be added to the active setif the additional energy of the base station (e.g. as measured by thepilot signal) and the energy of the base stations already in the activeset exceeds a predetermined threshold. Using this alternativeembodiment, a base station which transmitted energy comprises aninsubstantial amount of the total received energy at the mobile stationis not added to the active set.

[0024] It is yet another object of the present invention for the mobilestations to transmit the data rate requests on the DRC channel in amanner such that only the selected base station among the base stationsin communication with the mobile station is able to distinguish the DRCmessages, therefore assuring that the forward link transmission at anygiven time slot is from the selected base station. In the exemplaryembodiment, each base station in communication with the mobile stationis assigned a unique Walsh code. The mobile station covers the DRCmessage with the Walsh code corresponding to the selected base station.Other codes can be used to cover the DRC messages, although orthogonalcodes are typically utilized and Walsh codes are preferred.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The features, objects, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

[0026]FIG. 1 is a diagram of a data communication system of the presentinvention comprising a plurality of cells, a plurality of base stationsand a plurality of mobile stations.

[0027]FIG. 2 is an exemplary block diagram of the subsystems of the datacommunication system of the present invention;

[0028] FIGS. 3A-3B are block diagrams of the exemplary forward linkarchitecture of the present invention;

[0029]FIG. 4A is a diagram of the exemplary forward link frame structureof the present invention;

[0030] FIGS. 4B-4C are diagrams of the exemplary forward traffic channeland power control channel, respectively;

[0031]FIG. 4D is a diagram of the punctured packet of the presentinvention;

[0032] FIGS. 4E-4G are diagrams of the two exemplary data packet formatsand the control channel capsule, respectively;

[0033]FIG. 5 is an exemplary timing diagram showing the high rate packettransmission on the forward link;

[0034]FIG. 6 is a block diagram of the exemplary reverse linkarchitecture of the present invention;

[0035]FIG. 7A is a diagram of the exemplary reverse link frame structureof the present invention;

[0036]FIGS. 7B is a diagram of the exemplary reverse link accesschannel;

[0037]FIG. 8 is an exemplary timing diagram showing the high rate datatransmission on the reverse link;

[0038]FIG. 9 is an exemplary state diagram showing the transitionsbetween the various operating states of the mobile station; and

[0039]FIG. 10 is a diagram of the cumulative distribution function (CDF)of the C/I distribution in an ideal hexagonal cellular layout.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] In accordance with the exemplary embodiment of the datacommunication system of the present invention, forward link datatransmission occurs from one base station to one mobile station (seeFIG. 1) at or near the maximum data rate which can be supported by theforward link and the system. Reverse link data communication can occurfrom one mobile station to one or more base stations. The calculation ofthe maximum data rate for forward link transmission is described indetail below. Data is partitioned into data packets, with each datapacket being transmitted over one or more time slots (or slots). At eachtime slot, the base station can direct data transmission to any mobilestation which is in communication with the base station..

[0041] Initially, the mobile station establishes communication with abase station using a predetermined access procedure. In this connectedstate, the mobile station can receive data and control messages from thebase station, and is able to transmit data and control messages to thebase station. The mobile station then monitors the forward link fortransmissions from the base stations in the active set of the mobilestation. The active set contains a list of base stations incommunication with the mobile station. Specifically, the mobile stationmeasures the signal-to-noise-and-interference ratio (C/I) of the forwardlink pilot from the base stations in the active set, as received at themobile station. If the received pilot signal is above a predeterminedadd threshold or below a predetermined drop threshold, the mobilestation reports this to the base station. Subsequent messages from thebase station direct the mobile station to add or delete the basestation(s) to or from its active set, respectively. The variousoperating states of the mobile station is described below.

[0042] If there is no data to send, the mobile station returns to anidle state and discontinues transmission of data rate information to thebase station(s). While the mobile station is in the idle state, themobile station monitors the control channel from one or more basestations in the active set for paging messages.

[0043] If there is data to be transmitted to the mobile station, thedata is sent by a central controller to all base stations in the activeset and stored in a queue at each base station. A paging message is thensent by one or more base stations to the mobile station on therespective control channels. The base station may transmit all suchpaging messages at the same time across several base stations in orderto ensure reception even when the mobile station is switching betweenbase stations. The mobile station demodulates and decodes the signals onone or more control channels to receive the paging messages.

[0044] Upon decoding the paging messages, and for each time slot untilthe data transmission is completed, the mobile station measures the C/Iof the forward link signals from the base stations in the active set, asreceived at the mobile station. The C/I of the forward link signals canbe obtained by measuring the respective pilot signals. The mobilestation then selects the best base station based on a set of parameters.The set of parameters can comprise the present and previous C/Imeasurements and the bit-error-rate or packet-error-rate. For example,the best base station can be selected based on the largest C/Imeasurement. The mobile station then identifies the best base stationand transmits to the selected base station a data request message(hereinafter referred to as the DRC message) on the data request channel(hereinafter referred to as the DRC channel). The DRC message cancontain the requested data rate or, alternatively, an indication of thequality of the forward link channel (e.g., the C/I measurement itself,the bit-error-rate, or the packet-error-rate). In the exemplaryembodiment, the mobile station can direct the transmission of the DRCmessage to a specific base station by the use of a Walsh code whichuniquely identifies the base station. The DRC message symbols areexclusively OR'ed (XOR) with the unique Walsh code. Since each basestation in the active set of the mobile station is identified by aunique Walsh code, only the selected base station which performs theidentical XOR operation as that performed by the mobile station, withthe correct Walsh code, can correctly decode the DRC message. The basestation uses the rate control information from each mobile station toefficiently transmit forward link data at the highest possible rate.

[0045] At each time slot, the base station can select any of the pagedmobile stations for data transmission. The base station then determinesthe data rate at which to transmit the data to the selected mobilestation based on the most recent value of the DRC message received fromthe mobile station. Additionally, the base station uniquely identifies atransmission to a particular mobile station by using a spreading codewhich is unique to that mobile station. In the exemplary embodiment,this spreading code is the long pseudo noise (PN) code which is definedby IS95 standard.

[0046] The mobile station, for which the data packet is intended,receives the data transmission and decodes the data packet. Each datapacket comprises a plurality of data units. In the exemplary embodiment,a data unit comprises eight information bits, although different dataunit sizes can be defined and are within the scope of the presentinvention. In the exemplary embodiment, each data unit is associatedwith a sequence number and the mobile stations are able to identifyeither missed or duplicative transmissions. In such events, the mobilestations communicate via the reverse link data channel the sequencenumbers of the missing data units. The base station controllers, whichreceive the data messages from the mobile stations, then indicate to allbase stations communicating with this particular mobile station whichdata units were not received by the mobile station. The base stationsthen schedule a retransmission of such data units.

[0047] Each mobile station in the data communication system cancommunicate with multiple base stations on the reverse link. In theexemplary embodiment, the data communication system of the presentinvention supports soft handoff and softer handoff on the reverse linkfor several reasons. First, soft handoff does not consume additionalcapacity on the reverse link but rather allows the mobile stations totransmit data at the minimum power level such that at least one of thebase stations can reliably decode the data. Second, reception of thereverse link signals by more base stations increases the reliability ofthe transmission and only requires additional hardware at the basestations.

[0048] In the exemplary embodiment, the forward link capacity of thedata transmission system of the present invention is determined by therate requests of the mobile stations. Additional gains in the forwardlink capacity can be achieved by using directional antennas and/oradaptive spatial filters. An exemplary method and apparatus forproviding directional transmissions are disclosed in copending U.S.patent application Ser. No. 08/575,049, entitled “METHOD AND APPARATUSFOR DETERMINING THE TRANSMISSION DATA RATE IN A MULTI-USER COMMUNICATIONSYSTEM”, filed Dec. 20, 1995, and U.S. patent application Ser. No.08/925,521, entitled “METHOD AND APPARATUS FOR PROVIDING ORTHOGONAL SPOTBEAMS, SECTORS, AND PICOCELLS”, filed Sep. 8, 1997, both assigned to theassignee of the present invention and incorporated by reference herein.

[0049] I. System Description

[0050] Referring to the figures, FIG. 1 represents the exemplary datacommunication system of the present invention which comprises multiplecells 2 a-2 g. Each cell 2 is serviced by a corresponding base station4. Various mobile stations 6 are dispersed throughout the datacommunication system. In the exemplary embodiment, each of mobilestations 6 communicates with at most one base station 4 on the forwardlink at each time slot but can be in communication with one or more basestations 4 on the reverse link, depending on whether the mobile station6 is in soft handoff. For example, base station 4 a transmits dataexclusively to mobile station 6 a, base station 4 b transmits dataexclusively to mobile station 6 b, and base station 4 c transmits dataexclusively to mobile station 6 c on the forward link at time slot n. InFIG. 1, the solid line with the arrow indicates a data transmission frombase station 4 to mobile station 6. A broken line with the arrowindicates that mobile station 6 is receiving the pilot signal, but nodata transmission, from base station 4. The reverse link communicationis not shown in FIG. 1 for simplicity.

[0051] As shown by FIG. 1, each base station 4 preferably transmits datato one mobile station 6 at any given moment. Mobile stations 6,especially those located near a cell boundary, can receive the pilotsignals from multiple base stations 4. If the pilot signal is above apredetermined threshold, mobile station 6 can request that base station4 be added to the active set of mobile station 6. In the exemplaryembodiment, mobile station 6 can receive data transmission from zero orone member of the active set.

[0052] A block diagram illustrating the basic subsystems of the datacommunication system of the present invention is shown in FIG. 2. Basestation controller 10 interfaces with packet network interface 24, PSTN30, and all base stations 4 in the data communication system (only onebase station 4 is shown in FIG. 2 for simplicity). Base stationcontroller 10 coordinates the communication between mobile stations 6 inthe data communication system and other users connected to packetnetwork interface 24 and PSTN 30. PSTN 30 interfaces with users throughthe standard telephone network (not shown in FIG. 2).

[0053] Base station controller 10 contains many selector elements 14,although only one is shown in FIG. 2 for simplicity. One selectorelement 14 is assigned to control the communication between one or morebase stations 4 and one mobile station 6. If selector element 14 has notbeen assigned to mobile station 6, call control processor 16 is informedof the need to page mobile station 6. Call control processor 16 thendirects base station 4 to page mobile station 6.

[0054] Data source 20 contains the data which is to be transmitted tomobile station 6. Data source 20 provides the data to packet networkinterface 24. Packet network interface 24 receives the data and routesthe data to selector element 14. Selector element 14 sends the data toeach base station 4 in communication with mobile station 6. Each basestation 4 maintains data queue 40 which contains the data to betransmitted to mobile station 6.

[0055] In the exemplary embodiment, on the forward link, a data packetrefers to a predetermined amount of data which is independent of thedata rate. The data packet is formatted with other control and codingbits and encoded. If data transmission occurs over multiple Walshchannels, the encoded packet is demultiplexed into parallel streams,with each stream transmitted over one Walsh channel.

[0056] The data is sent, in data packets, from data queue 40 to channelelement 42. For each data packet, channel element 42 inserts thenecessary control fields. The data packet, control fields, frame checksequence bits, and code tail bits comprise a formatted packet. Channelelement 42 then encodes one or more formatted packets and interleaves(or reorders) the symbols within the encoded packets. Next, theinterleaved packet is scrambled with a scrambling sequence, covered withWalsh covers, and spread with the long PN code and the short PN_(I) andPN_(Q) codes. The spread data is quadrature modulated, filtered, andamplified by a transmitter within RF unit 44. The forward link signal istransmitted over the air through antenna 46 on forward link 50.

[0057] At mobile station 6, the forward link signal is received byantenna 60 and routed to a receiver within front end 62. The receiverfilters, amplifies, quadrature demodulates, and quantizes the signal.The digitized signal is provided to demodulator (DEMOD) 64 where it isdespread with the long PN code and the short PN_(I) and PN_(Q) codes,decovered with the Walsh covers, and descrambled with the identicalscrambling sequence. The demodulated data is provided to decoder 66which performs the inverse of the signal processing functions done atbase station 4, specifically the de-interleaving, decoding, and framecheck functions. The decoded data is provided to data sink 68. Thehardware, as described above, supports transmissions of data, messaging,voice, video, and other communications over the forward link.

[0058] The system control and scheduling functions can be accomplishedby many implementations. The location of channel scheduler 48 isdependent on whether a centralized or distributed control/schedulingprocessing is desired. For example, for distributed processing, channelscheduler 48 can be located within each base station 4. Conversely, forcentralized processing, channel scheduler 48 can be located within basestation controller 10 and can be designed to coordinate the datatransmissions of multiple base stations 4. Other implementations of theabove described functions can be contemplated and are within the scopeof the present invention.

[0059] As shown in FIG. 1, mobile stations 6 are dispersed throughoutthe data communication system and can be in communication with zero orone base station 4 on the forward link. In the exemplary embodiment,channel scheduler 48 coordinates the forward link data transmissions ofone base station 4. In the exemplary embodiment, channel scheduler 48connects to data queue 40 and channel element 42 within base station 4and receives the queue size, which is indicative of the amount of datato transmit to mobile station 6, and the DRC messages from mobilestations 6. Channel scheduler 48 schedules high rate data transmissionsuch that the system goals of maximum data throughput and minimumtransmission delay are optimized.

[0060] In the exemplary embodiment, the data transmission is scheduledbased in part on the quality of the communication link. An exemplarycommunication system which selects the transmission rate based on thelink quality is disclosed in U.S. patent application Ser. No.08/741,320, entitled “METHOD AND APPARATUS FOR PROVIDING HIGH SPEED DATACOMMUNICATIONS IN A CELLULAR ENVIRONMENT”, filed Sep. 11, 1996, assignedto the assignee of the present invention and incorporated by referenceherein. In the present invention, the scheduling of the datacommunication can be based on additional considerations such as the GOSof the user, the queue size, the type of data, the amount of delayalready experienced, and the error rate of the data transmission. Theseconsiderations are described in detail in U.S. patent application Ser.No. 08/798,951, entitled “METHOD AND APPARATUS FOR FORWARD LINK RATESCHEDULING”, filed Feb. 11, 1997, and U.S. patent application Ser. No.______, entitled “METHOD AND APPARATUS FOR REVERSE LINK RATESCHEDULING”, filed Aug. 20, 1997, both are assigned to the assignee ofthe present invention and incorporated by reference herein. Otherfactors can be considered in scheduling data transmissions and arewithin the scope of the present invention.

[0061] The data communication system of the present invention supportsdata and message transmissions on the reverse link. Within mobilestation 6, controller 76 processes the data or message transmission byrouting the data or message to encoder 72. Controller 76 can beimplemented in a microcontroller, a microprocessor, a digital signalprocessing (DSP) chip, or an ASIC programmed to perform the function asdescribed herein.

[0062] In the exemplary embodiment, encoder 72 encodes the messageconsistent with the Blank and Burst signaling data format described inthe aforementioned U.S. Pat. No. 5,504,773. Encoder 72 then generatesand appends a set of CRC bits, appends a set of code tail bits, encodesthe data and appended bits, and reorders the symbols within the encodeddata. The interleaved data is provided to modulator (MOD) 74.

[0063] Modulator 74 can be implemented in many embodiments. In theexemplary embodiment (see FIG. 6), the interleaved data is covered withWalsh codes, spread with a long PN code, and further spread with theshort PN codes. The spread data is provided to a transmitter withinfront end 62. The transmitter modulates, filters, amplifies, andtransmits the reverse link signal over the air, through antenna 46, onreverse link 52.

[0064] In the exemplary embodiment, mobile station 6 spreads the reverselink data in accordance with a long PN code. Each reverse link channelis defined in accordance with the temporal offset of a common long PNsequence. At two differing offsets the resulting modulation sequencesare uncorrelated. The offset of a mobile station 6 is determined inaccordance with a unique numerical identification of mobile station 6,which in the exemplary embodiment of the IS-95 mobile stations 6 is themobile station specific identification number. Thus, each mobile station6 transmits on one uncorrelated reverse link channel determined inaccordance with its unique electronic serial number.

[0065] At base station 4, the reverse link signal is received by antenna46 and provided to RF unit 44. RF unit 44 filters, amplifies,demodulates, and quantizes the signal and provides the digitized signalto channel element 42. Channel element 42 despreads the digitized signalwith the short PN codes and the long PN code. Channel element 42 alsoperforms the Walsh code decovering and pilot and DRC extraction. Channelelement 42 then reorders the demodulated data, decodes thede-interleaved data, and performs the CRC check function. The decodeddata, e.g. the data or message, is provided to selector element 14.Selector element 14 routes the data and message to the appropriatedestination. Channel element 42 may also forward a quality indicator toselector element 14 indicative of the condition of the received datapacket.

[0066] In the exemplary embodiment, mobile station 6 can be in one ofthree operating states. An exemplary state diagram showing thetransitions between the various operating states of mobile station 6 isshown in FIG. 9. In the access state 902, mobile station 6 sends accessprobes and waits for channel assignment by base station 4. The channelassignment comprises allocation of resources, such as a power controlchannel and frequency allocation. Mobile station 6 can transition fromthe access state 902 to the connected state 904 if mobile station 6 ispaged and alerted to an upcoming data transmission, or if mobile station6 transmits data on the reverse link. In the connected state 904, mobilestation 6 exchanges (e.g., transmits or receives) data and performshandoff operations. Upon completion of a release procedure, mobilestation 6 transitions from the connected state 904 to the idle state906. Mobile station 6 can also transmission from the access state 902 tothe idle state 906 upon being rejected of a connection with base station4. In the idle state 906, mobile station 6 listens to overhead andpaging messages by receiving and decoding messages on the forwardcontrol channel and performs idle handoff procedure. Mobile station 6can transition to the access state 902 by initiating xxx (Matt what isthis procedure called???) procedure. The state diagram shown in FIG. 9is only an exemplary state definition which is shown for illustration.Other state diagrams can also be utilized and are within the scope ofthe present invention.

[0067] II. Forward Link Data Transmission

[0068] In the exemplary embodiment, the initiation of a communicationbetween mobile station 6 and base station 4 occurs in a similar manneras that for the CDMA system. After completion of the call set up, mobilestation 6 monitors the control channel for paging messages. While in theconnected state, mobile station 6 begins transmission of the pilotsignal on the reverse link.

[0069] An exemplary flow diagram of the forward link high rate datatransmission of the present invention is shown in FIG. 5. If basestation 4 has data to transmit to mobile station 6, base station 4 sendsa paging message addressed to mobile station 6 on the control channel atblock 502. The paging message can be sent from one or multiple basestations 4, depending on the handoff state of mobile station 6. Uponreception of the paging message, mobile station 6 begins the C/Imeasurement process at block 504. The C/I of the forward link signal iscalculated from one or a combination of methods described below. Mobilestation 6 then selects a requested data rate based on the best C/Imeasurement and transmits a DRC message on the DRC channel at block 506.

[0070] Within the same time slot, base station 4 receives the DRCmessage at block 508. If the next time slot is available for datatransmission, base station 4 transmits data to mobile station 6 at therequested data rate at block 510. Mobile station 6 receives the datatransmission at block 512. If the next time slot is available, basestation 4 transmits the remainder of the packet at block 514 and mobilestation 6 receives the data transmission at block 516.

[0071] In the present invention, mobile station 6 can be incommunication with one or more base stations 4 simultaneously. Theactions taken by mobile station 6 depend on whether mobile station 6 isor is not in soft handoff. These two cases are discussed separatelybelow.

[0072] III. No Handoff Case

[0073] In the no handoff case, mobile station 6 communicates with onebase station 4. Referring to FIG. 2, the data destined for a particularmobile station 6 is provided to selector element 14 which has beenassigned to control the communication with that mobile station 6.Selector element 14 forwards the data to data queue 40 within basestation 4. Base station 4 queues the data and transmits a paging messageon the control channel. Base station 4 then monitors the reverse linkDRC channel for DRC messages from mobile station 6. If no signal isdetected on the DRC channel, base station 4 can retransmit the pagingmessage until the DRC message is detected. After a predetermined numberof retransmission attempts, base station 4 can terminate the process orre-initiate a call with mobile station 6.

[0074] In the exemplary embodiment, mobile station 6 transmits therequested data rate, in the form of a DRC message, to base station 4 onthe DRC channel. In the alternative embodiment, mobile station 6transmits an indication of the quality of the forward link channel(e.g., the C/I measurement) to base station 4. In the exemplaryembodiment, the 3-bit DRC message is decoded with soft decisions by basestation 4. In the exemplary embodiment, the DRC message is transmittedwithin the first half of each time slot. Base station 4 then has theremaining half of the time slot to decode the DRC message and configurethe hardware for data transmission at the next successive time slot, ifthat time slot is available for data transmission to this mobile station6. If the next successive time slot is not available, base station 4waits for the next available time slot and continues to monitor the DRCchannel for the new DRC messages.

[0075] In the first embodiment, base station 4 transmits at therequested data rate. This embodiment confers to mobile station 6 theimportant decision of selecting the data rate. Always transmitting atthe requested data rate has the advantage that mobile station 6 knowswhich data rate to expect. Thus, mobile station 6 only demodulates anddecodes the traffic channel in accordance with the requested data rate.Base station 4 does not have to transmit a message to mobile station 6indicating which data rate is being used by base station 4.

[0076] In the first embodiment, after reception of the paging message,mobile station 6 continuously attempts to demodulate the data at therequested data rate. Mobile station 6 demodulates the forward trafficchannel and provides the soft decision symbols to the decoder. Thedecoder decodes the symbols and performs the frame check on the decodedpacket to determine whether the packet was received correctly. If thepacket was received in error or if the packet was directed for anothermobile station 6, the frame check would indicate a packet error.Alternatively in the first embodiment, mobile station 6 demodulates thedata on a slot by slot basis. In the exemplary embodiment, mobilestation 6 is able to determine whether a data transmission is directedfor it based on a preamble which is incorporated within each transmitteddata packet, as described below. Thus, mobile station 6 can terminatethe decoding process if it is determined that the transmission isdirected for another mobile station 6. In either case, mobile station 6transmits a negative acknowledgments (NACK) message to base station 4 toacknowledge the incorrect reception of the data units. Upon receipt ofthe NACK message, the data units received in error is retransmitted.

[0077] The transmission of the NACK messages can be implemented in amanner similar to the transmission of the error indicator bit (EIB) inthe CDMA system. The implementation and use of EIB transmission aredisclosed in U.S. Pat. No. 5,568,483, entitled “METHOD AND APPARATUS FORTHE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assignee ofthe present invention and incorporated by reference herein.Alternatively, NACK can be transmitted with messages.

[0078] In the second embodiment, the data rate is determined by basestation 4 with input from mobile station 6. Mobile station 6 performsthe C/I measurement and transmits an indication of the link quality(e.g., the C/I measurement) to base station 4. Base station 4 can adjustthe requested data rate based on the resources available to base station4, such as the queue size and the available transmit power. The adjusteddata rate can be transmitted to mobile station 6 prior to or concurrentwith data transmission at the adjusted data rate, or can be implied inthe encoding of the data packets. In the first case, wherein mobilestation 6 receives the adjusted data rate before the data transmission,mobile station 6 demodulates and decodes the received packet in themanner described in the first embodiment. In the second case, whereinthe adjusted data rate is transmitted to mobile station 6 concurrentwith the data transmission, mobile station 6 can demodulate the forwardtraffic channel and store the demodulated data. Upon receipt of theadjusted data rate, mobile station 6 decodes the data in accordance withthe adjusted data rate. And in the third case, wherein the adjusted datarate is implied in the encoded data packets, mobile station 6demodulates and decodes all candidate rates and determine aposteriorithe transmit rate for selection of the decoded data. The method andapparatus for performing rate determination are described in detail inU.S. patent application Ser. No. 08/730,863, entitled “METHOD ANDAPPARATUS FOR DETERMINING THE RATE OF RECEIVED DATA IN A VARIABLE RATECOMMUNICATION SYSTEM”, filed Oct. 18, 1996, and patent application Ser.No. PA436, also entitled “METHOD AND APPARATUS FOR DETERMINING THE RATEOF RECEIVED DATA IN A VARIABLE RATE COMMUNICATION SYSTEM”, filed ______,both assigned to the assignee of the present invention and incorporatedby reference herein. For all cases described above, mobile station 6transmits a NACK message as described above if the outcome of the framecheck is negative.

[0079] The discussion hereinafter is based on the first embodimentwherein mobile station 6 transmits to base station 4 the DRC messageindicative of the requested data rate, except as otherwise indicated.However, the inventive concept described herein is equally applicable tothe second embodiment wherein mobile station 6 transmits an indicationof the link quality to base station 4.

[0080] IV. Handoff Case

[0081] In the handoff case, mobile station 6 communicates with multiplebase stations 4 on the reverse link. In the exemplary embodiment, datatransmission on the forward link to a particular mobile station 6 occursfrom one base station 4. However, mobile station 6 can simultaneouslyreceive the pilot signals from multiple base stations 4. If the C/Imeasurement of a base station 4 is above a predetermined threshold, thebase station 4 is added to the active set of mobile station 6. Duringthe soft handoff direction message, the new base station 4 assignsmobile station 6 to a reverse power control (RPC) Walsh channel which isdescribed below. Each base station 4 in soft handoff with mobile station6 monitors the reverse link transmission and sends an RPC bit on theirrespective RPC Walsh channels.

[0082] Referring to FIG. 2, selector element 14 assigned to control thecommunication with mobile station 6 forwards the data to all basestations 4 in the active set of mobile station 6. All base stations 4which receive data from selector element 14 transmit a paging message tomobile station 6 on their respective control channels. When mobilestation 6 is in the connected state, mobile station 6 performs twofunctions. First, mobile station 6 selects the best base station 4 basedon a set of parameter which can be the best C/I measurement. Mobilestation 6 then selects a data rate corresponding to the C/I measurementand transmits a DRC message to the selected base station 4. Mobilestation 6 can direct transmission of the DRC message to a particularbase station 4 by covering the DRC message with the Walsh cover assignedto that particular base station 4. Second, mobile station 6 attempts todemodulate the forward link signal in accordance with the requested datarate at each subsequent time slot.

[0083] After transmitting the paging messages, all base stations 4 inthe active set monitor the DRC channel for a DRC message from mobilestation 6. Again, because the DRC message is covered with a Walsh code,the selected base station 4 assigned with the identical Walsh cover isable to decover the DRC message. Upon receipt of the DRC message, theselected base station 4 transmits data to mobile station 6 at the nextavailable time slots.

[0084] In the exemplary embodiment, base station 4 transmits data inpackets comprising a plurality of data units at the requested data rateto mobile station 6. If the data units are incorrectly received bymobile station 6, a NACK message is transmitted on the reverse links toall base stations 4 in the active set. In the exemplary embodiment, theNACK message is demodulated and decoded by base stations 4 and forwardedto selector element 14 for processing. Upon processing of the NACKmessage, the data units are retransmitted using the procedure asdescribed above. In the exemplary embodiment, selector element 14combines the NACK signals received from all base stations 4 into oneNACK message and sends the NACK message to all base stations 4 in theactive set.

[0085] In the exemplary embodiment, mobile station 6 can detect changesin the best C/I measurement and dynamically request data transmissionsfrom different base stations 4 at each time slot to improve efficiency.In the exemplary embodiment, since data transmission occurs from onlyone base station 4 at any given time slot, other base stations 4 in theactive set may not be aware which data units, if any, has beentransmitted to mobile station 6. In the exemplary embodiment, thetransmitting base station 4 informs selector element 14 of the datatransmission. Selector element 14 then sends a message to all basestations 4 in the active set. In the exemplary embodiment, thetransmitted data is presumed to have been correctly received by mobilestation 6. Therefore, if mobile station 6 requests data transmissionfrom a different base station 4 in the active set, the new base station4 transmits the remaining data units. In the exemplary embodiment, thenew base station 4 transmits in accordance with the last transmissionupdate from selector element 14. Alternatively, the new base station 4selects the next data units to transmit using predictive schemes basedon metrics such as the average transmission rate and prior updates fromselector element 14. These mechanisms minimize duplicativeretransmissions of the same data units by multiple base stations 4 atdifferent time slots which results in a loss in efficiency. If a priortransmission was received in error, base stations 4 can retransmit thosedata units out of sequence since each data unit is identified by aunique sequence number as described below. In the exemplary embodiment,if a hole (or non-transmitted data units) is created (e.g., as theresult of handoff between one base station 4 to another base station 4),the missing data units are considered as though received in error.Mobile station 6 transmits NACK messages corresponding to the missingdata units and these data units are retransmitted.

[0086] In the exemplary embodiment, each base station 4 in the activeset maintains an independent data queue 40 which contains the data to betransmitted to mobile station 6. The selected base station 4 transmitsdata existing in its data queue 40 in a sequential order, except forretransmissions of data units received in error and signaling messages.In the exemplary embodiment, the transmitted data units are deleted fromqueue 40 after transmission.

[0087] V. Other Considerations for Forward Link Data Transmissions

[0088] An important consideration in the data communication system ofthe present invention is the accuracy of the C/I estimates for thepurpose of selecting the data rate for future transmissions. In theexemplary embodiment, the C/I measurements are performed on the pilotsignals during the time interval when base stations 4 transmit pilotsignals. In the exemplary embodiment, since only the pilot signals aretransmitted during this pilot time interval, the effects of multipathand interference are minimal.

[0089] In other implementations of the present invention wherein thepilot signals are transmitted continuously over an orthogonal codechannel, similar to that for the IS-95 systems, the effect of multipathand interference can distort the C/I measurements. Similarly, whenperforming the C/I measurement on the data transmissions instead of thepilot signals, multipath and interference can also degrade the C/Imeasurements. In both of these cases, when one base station 4 istransmitting to one mobile station 6, the mobile station 6 is able toaccurately measure the C/I of the forward link signal because no otherinterfering signals are present. However, when mobile station 6 is insoft handoff and receives the pilot signals from multiple base stations4, mobile station 6 is not able to discern whether or not base stations4 were transmitting data. In the worst case scenario, mobile station 6can measure a high C/I at a first time slot, when no base stations 4were transmitting data to any mobile station 6, and receive datatransmission at a second time slot, when all base stations 4 aretransmitting data at the same time slot. The C/I measurement at thefirst time slot, when all base stations 4 are idle, gives a falseindication of the forward link signal quality at the second time slotsince the status of the data communication system has changed. In fact,the actual C/I at the second time slot can be degraded to the point thatreliable decoding at the requested data rate is not possible.

[0090] The converse extreme scenario exists when a C/I estimate bymobile station 6 is based on maximal interference. However, the actualtransmission occurs when only the selected base station is transmitting,In this case, the C/I estimate and selected data rate are conservativeand the transmission occurs at a rate lower than that which could bereliably decoded, thus reducing the transmission efficiency.

[0091] In the implementation wherein the C/I measurement is performed ona continuous pilot signal or the traffic signal, the prediction of theC/I at the second time slot based on the measurement of the C/I at thefirst time slot can be made more accurate by three embodiments. In thefirst embodiment, data transmissions from base stations 4 are controlledso that base stations 4 do not constantly toggle between the transmitand idle states at successive time slots. This can be achieved byqueuing enough data (e.g. a predetermined number of information bits)before actual data transmission to mobile stations 6.

[0092] In the second embodiment, each base station 4 transmits a forwardactivity bit (hereinafter referred to as the FAC bit) which indicateswhether a transmission will occur at the next half frame. The use of theFAC bit is described in detail below. Mobile station 6 performs the C/Imeasurement taking into account the received FAC bit from each basestation 4.

[0093] In the third embodiment, which corresponds to the scheme whereinan indication of the link quality is transmitted to base station 4 andwhich uses a centralized scheduling scheme, the scheduling informationindicating which ones of base stations 4 transmitted data at each timeslot is made available to channel scheduler 48. Channel scheduler 48receives the C/I measurements from mobile stations 6 and can adjust theC/I measurements based on its knowledge of the presence or absence ofdata transmission from each base station 4 in the data communicationsystem. For example, mobile station 6 can measure the C/I at the firsttime slot when no adjacent base stations 4 are transmitting. Themeasured C/I is provided to channel scheduler 48. Channel scheduler 48knows that no adjacent base stations 4 transmitted data in the firsttime slot since none was scheduled by channel scheduler 48. Inscheduling data transmission at the second time slot, channel scheduler48 knows whether one or more adjacent base stations 4 will transmitdata. Channel scheduler 48 can adjust the C/I measured at the first timeslot to take into account the additional interference mobile station 6will receive in the second time slot due to data transmissions byadjacent base stations 4. Alternately, if the C/I is measured at thefirst time slot when adjacent base stations 4 are transmitting and theseadjacent base stations 4 are not transmitting at the second time slot,channel scheduler 48 can adjust the C/I measurement to take into accountthe additional information.

[0094] Another important consideration is to minimize redundantretransmissions. Redundant retransmissions can result from allowingmobile station 6 to select data transmission from different basestations 4 at successive time slots. The best C/I measurement can togglebetween two or more base stations 4 over successive time slots if mobilestation 6 measures approximately equal C/I for these base stations 4.The toggling can be due to deviations in the C/I measurements and/orchanges in the channel condition. Data transmission by different basestations 4 at successive time slots can result in a loss in efficiency.

[0095] The toggling problem can be addressed by the use of hysterisis.The hysterisis can be implemented with a signal level scheme, a timingscheme, or a combination of the signal level and timing schemes. In theexemplary signal level scheme, the better C/I measurement of a differentbase station 4 in the active set is not selected unless it exceeds theC/I measurement of the current transmitting base station 4 by at leastthe hysterisis quantity. As an example, assume that the hysterisis is1.0 dB and that the C/I measurement of the first base station 4 is 3.5dB and the C/I measurement of the second base station 4 is 3.0 dB at thefirst time slot. At the next time slot, the second base station 4 is notselected unless its C/I measurement is at least 1.0 dB higher than thatof the first base station 4. Thus, if the C/I measurement of the firstbase station 4 is still 3.5 dB at the next time slot, the second basestation 4 is not selected unless its C/I measurement is at least 4.5 dB.

[0096] In the exemplary timing scheme, base station 4 transmits datapackets to mobile station 6 for a predetermined number of time slots.Mobile station 6 is not allowed to select a different transmitting basestation 4 within the predetermined number of time slots. Mobile station6 continues to measure the C/I of the current transmitting base station4 at each time slot and selects the data rate in response to the C/Imeasurement.

[0097] Yet another important consideration is the efficiency of the datatransmission. Referring to FIGS. 4E and 4F, each data packet format 410and 430 contains data and overhead bits. In the exemplary embodiment,the number of overhead bits is fixed for all data rates. At the highestdata rate, the percentage of overhead is small relative to the packetsize and the efficiency is high. At the lower data rates, the overheadbits can comprise a larger percentage of the packet. The inefficiency atthe lower data rates can be improved by transmitting variable lengthdata packets to mobile station 6. The variable length data packets canbe partitioned and transmitted to mobile station 6 over multiple timeslots. Preferably, the variable length data packets are transmitted tomobile station 6 over successive time slots to simplify the processing.The present invention is directed to the use of various packet sizes forvarious supported data rates to improve the overall transmissionefficiency.

[0098] VI. Forward Link Architecture

[0099] In the exemplary embodiment, base station 4 transmits at themaximum power available to base station 4 and at the maximum data ratesupported by the data communication system to a single mobile station 6at any given slot. The maximum data rate that can be supported isdynamic and depends on the C/I of the forward link signal as measured bymobile station 6. Preferably, base station 4 transmits to only onemobile station 6 at any given time slot.

[0100] To facilitate data transmission, the forward link comprises fourtime multiplexed channels: the pilot channel, power control channel,control channel, and traffic channel. The function and implementation ofeach of these channels are described below. In the exemplary embodiment,the traffic and power control channels each comprises a number oforthogonally spread Walsh channels. In the present invention, thetraffic channel is used to transmit traffic data and paging messages tomobile stations 6. When used to transmit paging messages, the trafficchannel is also referred to as the control channel in thisspecification.

[0101] In the exemplary embodiment, the bandwidth of the forward link isselected to be 1.2288 MHz. This bandwidth selection allows the use ofexisting hardware components designed for a CDMA system which conformsto the IS-95 standard. However, the data communication system of thepresent invention can be adopted for use with different bandwidths toimprove capacity and/or to conform to system requirements. For example,a 5 MHz bandwidth can be utilized to increase the capacity. Furthermore,the bandwidths of the forward link and the reverse link can be different(e.g., 5 MHz bandwidth on the forward link and 1.2288 MHz bandwidth onthe reverse link) to more closely match link capacity with demand.

[0102] In the exemplary embodiment, the short PN_(I) and PN_(Q) codesare the same length 2¹⁵ PN codes which are specified by the IS-95standard. At the 1.2288 MHz chip rate, the short PN sequences repeatevery 26.67 msec {26.67 msec=2¹⁵/1.2288×10⁶}. In the exemplaryembodiment, the same short PN codes are used by all base stations 4within the data communication system. However, each base station 4 isidentified by a unique offset of the basic short PN sequences. In theexemplary embodiment, the offset is in increments of 64 chips. Otherbandwidth and PN codes can be utilized and are within the scope of thepresent invention.

[0103] VI. Forward Link Traffic Channel

[0104] A block diagram of the exemplary forward link architecture of thepresent invention is shown in FIG. 3A. The data is partitioned into datapackets and provided to CRC encoder 112. For each data packet, CRCencoder 112 generates frame check bits (e.g., the CRC parity bits) andinserts the code tail bits. The formatted packet from CRC encoder 112comprises the data, the frame check and code tail bits, and otheroverhead bits which are described below. The formatted packet isprovided to encoder 114 which, in the exemplary embodiment, encodes thepacket in accordance with the encoding format disclosed in theaforementioned U.S. patent application Ser. No. 08/743,688. Otherencoding formats can also be used and are within the scope of thepresent invention. The encoded packet from encoder 114 is provided tointerleaver 116 which reorders the code symbols in the packet. Theinterleaved packet is provided to frame puncture element 118 whichremoves a fraction of the packet in the manner described below. Thepunctured packet is provided to multiplier 120 which scrambles the datawith the scrambling sequence from scrambler 122. Puncture element 118and scrambler 122 are described in detail below. The output frommultiplier 120 comprises the scrambled packet.

[0105] The scrambled packet is provided to variable rate controller 130which demultiplexes the packet into K parallel inphase and quadraturechannels, where K is dependent on the data rate. In the exemplaryembodiment, the scrambled packet is first demultiplexed into the inphase(I) and quadrature (Q) streams. In the exemplary embodiment, the Istream comprises even indexed symbols and the Q stream comprises oddindexed symbol. Each stream is further demultiplexed into K parallelchannels such that the symbol rate of each channel is fixed for all datarates. The K channels of each stream are provided to Walsh cover element132 which covers each channel with a Walsh function to provideorthogonal channels. The orthogonal channel data are provided to gainelement 134 which scales the data to maintain a constanttotal-energy-per-chip (and hence constant output power) for all datarates. The scaled data from gain element 134 is provided to multiplexer(MUX) 160 which multiplexes the data with the preamble. The preamble isdiscussed in detail below. The output from MUX 160 is provided tomultiplexer (MUX) 162 which multiplexes the traffic data, the powercontrol bits, and the pilot data. The output of MUX 162 comprises the IWalsh channels and the Q Walsh channels.

[0106] A block diagram of the exemplary modulator used to modulate thedata is illustrated in FIG. 3B. The I Walsh channels and Q Walshchannels are provided to summers 212 a and 212 b, respectively, whichsum the K Walsh channels to provide the signals I_(sum) and Q_(sum),respectively. The I_(sum) and Q_(sum) signals are provided to complexmultiplier 214. Complex multiplier 214 also receives the PN_I and PN_Qsignals from multipliers 236 a and 236 b, respectively, and multipliesthe two complex inputs in accordance with the following equation:$\begin{matrix}{\begin{matrix}{\left( {I_{mult} + {jQ}_{mult}} \right) = \quad {\left( {I_{sum} + {jQ}_{sum}} \right) \cdot \left( {{PN\_ I} + {jPN\_ Q}} \right)}} \\{= \quad {\left( {{I_{sum} \cdot {PN\_ I}} - {Q_{sum} \cdot {PN\_ Q}}} \right) +}} \\{\quad {{j\left( {{I_{sum} \cdot {PN\_ Q}} + {Q_{sum} \cdot {PN\_ I}}} \right)},}}\end{matrix}\quad} & (2)\end{matrix}$

[0107] where I_(mult) and Q_(mult) are the outputs from complexmultiplier 214 and j is the complex representation. The I_(mult) andQ_(mult) signals are provided to filters 216 a and 216 b, respectively,which filters the signals. The filtered signals from filters 216 a and216 b are provided to multipliers 218 a and 218 b, respectively, whichmultiplies the signals with the inphase sinusoid COS(w_(c)t) and thequadrature sinusoid SIN(w_(c)t), respectively. The I modulated and Qmodulated signals are provided to summer 220 which sums the signals toprovide the forward modulated waveform S(t).

[0108] In the exemplary embodiment, the data packet is spread with thelong PN code and the short PN codes. The long PN code scrambles thepacket such that only the mobile station 6 for which the packet isdestined is able to descramble the packet. In the exemplary embodiment,the pilot and power control bits and the control channel packet arespread with the short PN codes but not the long PN code to allow allmobile stations 6 to receive these bits. The long PN sequence isgenerated by long code generator 232 and provided to multiplexer (MUX)234. The long PN mask determines the offset of the long PN sequence andis uniquely assigned to the destination mobile station 6. The outputfrom MUX 234 is the long PN sequence during the data portion of thetransmission and zero otherwise (e.g. during the pilot and power controlportion). The gated long PN sequence from MUX 234 and the short PN_(I)and PN_(Q) sequences from short code generator 238 are provided tomultipliers 236 a and 236 b, respectively, which multiply the two setsof sequences to form the PN_I and PN_Q signals, respectively. The PN_Iand PN_Q signals are provided to complex multiplier 214.

[0109] The block diagram of the exemplary traffic channel shown in FIGS.3A and 3B is one of numerous architectures which support data encodingand modulation on the forward link. Other architectures, such as thearchitecture for the forward link traffic channel in the CDMA systemwhich conforms to the IS-95 standard, can also be utilized and arewithin the scope of the present invention.

[0110] In the exemplary embodiment, the data rates supported by basestations 4 are predetermined and each supported data rate is assigned aunique rate index. Mobile station 6 selects one of the supported datarates based on the C/I measurement. Since the requested data rate needsto be sent to a base station 4 to direct that base station 4 to transmitdata at the requested data rate, a trade off is made between the numberof supported data rates and the number of bits needed to identify therequested data rate. In the exemplary embodiment, the number ofsupported data rates is seven and a 3-bit rate index is used to identifythe requested data rate. An exemplary definition of the supported datarates is illustrated in Table 1. Different definition of the supporteddata rates can be contemplated and are within the scope of the presentinvention.

[0111] In the exemplary embodiment, the minimum data rate is 38.4 Kbpsand the maximum data rate is 2.4576 Mbps. The minimum data rate isselected based on the worse case C/I measurement in the system, theprocessing gain of the system, the design of the error correcting codes,and the desired level of performance. In the exemplary embodiment, thesupported data rates are chosen such that the difference betweensuccessive supported data rates is 3 dB. The 3 dB increment is acompromise among several factors which include the accuracy of the C/Imeasurement that can be achieved by mobile station 6, the losses (orinefficiencies) which results from the quantization of the data ratesbased on the C/I measurement, and the number of bits (or the bit rate)needed to transmit the requested data rate from mobile station 6 to basestation 4. More supported data rates requires more bits to identify therequested data rate but allows for more efficient use of the forwardlink because of smaller quantization error between the calculatedmaximum data rate and the supported data rate. The present invention isdirected to the use of any number of supported data rates and other datarates than those listed in Table 1. TABLE 1 Traffic Channel ParametersData Rates Units Parameter 38.4 76.8 153.6 307.2 614.4 1228.8 2457.6Kbps Data bit/packet 1024 1024 1024 1024 1024 2048 2048 bits Packetlength 26.67 13.33 6.67 3.33 1.67 1.67 0.83 msec Slots/packet 16 8 4 2 11 0.5 slots Packet/transmission 1 1 1 1 1 1 2 packets Slots/transmission16 8 4 2 1 1 1 slots Walsh symbol rate 153.6 307.2 614.4 1228.8 2457.62457.6 4915.2 Ksps Walsh channel/ 1 2 4 8 16 16 16 channels QPSK phaseModulator rate 76.8 76.8 76.8 76.8 76.8 76.8 76.8¹ ksps PN chips/databit 32 16 8 4 2 1 0.5 chips/bit PN chip rate 1228.8 1228.8 1228.8 1228.81228.8 1228.8 1228.8 Kcps Modulation format QPSK QPSK QPSK QPSK QPSKQPSK QAM¹ Rate index 0 1 2 3 4 5 6

[0112] A diagram of the exemplary forward link frame structure of thepresent invention is illustrated in FIG. 4A. The traffic channeltransmission is partitioned into frames which, in the exemplaryembodiment, are defined as the length of the short PN sequences or 26.67msec. Each frame can carry control channel information addressed to allmobile stations 6 (control channel frame), traffic data addressed to aparticular mobile station 6 (traffic frame), or can be empty (idleframe). The content of each frame is determined by the schedulingperformed by the transmitting base station 4. In the exemplaryembodiment, each frame comprises 16 time slots, with each time slothaving a duration of 1.667 msec. A time slot of 1.667 msec is adequateto enable mobile station 6 to perform the C/I measurement of the forwardlink signal. A time slot of 1.667 msec also represents a sufficientamount of time for efficient packet data transmission. In the exemplaryembodiment, each time slot is further partitioned into four quarterslots.

[0113] In the present invention, each data packet is transmitted overone or more time slots as shown in Table 1. In the exemplary embodiment,each forward link data packet comprises 1024 or 2048 bits. Thus, thenumber of time slots required to transmit each data packet is dependenton the data rate and ranges from 16 time slots for the 38.4 Kbps rate to1 time slot for the 1.2288 Mbps rate and higher.

[0114] An exemplary diagram of the forward link slot structure of thepresent invention is shown in FIG. 4B. In the exemplary embodiment, eachslot comprises three of the four time multiplexed channels, the trafficchannel, the control channel, the pilot channel, and the power controlchannel. In the exemplary embodiment, the pilot and power controlchannels are transmitted in two pilot and power control bursts which arelocated at the same positions in each time slot. The pilot and powercontrol bursts are described in detail below.

[0115] In the exemplary embodiment, the interleaved packet frominterleaver 116 is punctured to accommodate the pilot and power controlbursts. In the exemplary embodiment, each interleaved packet comprises4096 code symbols and the first 512 code symbols are punctured, as shownin FIG. 4D. The remaining code symbols are skewed in time to align tothe traffic channel transmission intervals.

[0116] The punctured code symbols are scrambled to randomize the dataprior to applying the orthogonal Walsh cover. The randomization limitsthe peak-to-average envelope on the modulated waveform S(t). Thescrambling sequence can be generated with a linear feedback shiftregister, in a manner known in the art. In the exemplary embodiment,scrambler 122 is loaded with the LC state at the start of each slot. Inthe exemplary embodiment, the clock of scrambler 122 is synchronous withthe clock of interleaver 116 but is stalled during the pilot and powercontrol bursts.

[0117] In the exemplary embodiment, the forward Walsh channels (for thetraffic channel and power control channel) are orthogonally spread with16-bit Walsh covers at the fixed chip rate of 1.2288 Mcps. The number ofparallel orthogonal channels K per inphase and quadrature signal is afunction of the data rate, as shown in Table 1. In the exemplaryembodiment, for lower data rates, the inphase and quadrature Walshcovers are chosen to be orthogonal sets to minimize cross-talk to thedemodulator phase estimate errors. For example, for 16 Walsh channels,an exemplary Walsh assignment is W₀ through W₇ for the inphase signaland W₈ through W₁₅ for the quadrature signal.

[0118] In the exemplary embodiment, QPSK modulation is used for datarates of 1.2288 Mbps and lower. For QPSK modulation, each Walsh channelcomprises one bit. In the exemplary embodiment, at the highest data rateof 2.4576 Mbps, 16-QAM is used and the scrambled data is demultiplexedinto 32 parallel streams which are each 2-bit wide, 16 parallel streamsfor the inphase signal and 16 parallel streams for the quadraturesignal. In the exemplary embodiment, the LSB of each 2-bit symbol is theearlier symbol output from interleaver 116. In the exemplary embodiment,the QAM modulation inputs of (0, 1, 3, 2) map to modulation values of(+3, +1, −1, −3), respectively. The use of other modulation schemes,such as m-ary phase shift keying PSK, can be contemplated and are withinthe scope of the present invention.

[0119] The inphase and quadrature Walsh channels are scaled prior tomodulation to maintain a constant total transmit power which isindependent of the data rate. The gain settings are normalized to aunity reference equivalent to unmodulated BPSK. The normalized channelgains G as a function of the number of Walsh channels (or data rate) areshown in Table 2. Also listed in Table 2 is the average power per Walshchannel (inphase or quadrature) such that the total normalized power isequal to unity. Note that the channel gain for 16-QAM accounts for thefact that the normalized energy per Walsh chip is 1 for QPSK and 5 for16-QAM. TABLE 2 Traffic Channel Orthogonal Channel Gains PunctureDuration Number of Walsh Average Data Rate Walsh Channel Power per(Kbps) Channels K Modulation Gain G Channel P_(k) 38.4 1 QPSK 1/{squareroot}{square root over (2)} ½ 76.8 2 QPSK ½ ¼ 153.6 4 QPSK 1/2{squareroot}{square root over (2)} ⅛ 307.2 8 QPSK ¼ {fraction (1/16)} 614.4 16QPSK 1/4{square root}{square root over (2)} {fraction (1/32)} 1228.8 16QPSK 1/4{square root}{square root over (2)} {fraction (1/32)} 2457.6 1616-QAM 1/4{square root}{square root over (10)} {fraction (1/32)}

[0120] In the present invention, a preamble is punctured into eachtraffic frame to assist mobile station 6 in the synchronization with thefirst slot of each variable rate transmission. In the exemplaryembodiment, the preamble is an all-zero sequence which, for a trafficframe, is spread with the long PN code but, for a control channel frame,is not spread with the long PN code. In the exemplary embodiment, thepreamble is unmodulated BPSK which is orthogonally spread with Walshcover W₁. The use of a single orthogonal channel minimizes thepeak-to-average envelope. Also, the use of a non-zero Walsh cover W₁minimizes false pilot detection since, for traffic frames, the pilot isspread with Walsh cover W₀ and both the pilot and the preamble are notspread with the long PN code.

[0121] The preamble is multiplexed into the traffic channel stream atthe start of the packet for a duration which is a function of the datarate. The length of the preamble is such that the preamble overhead isapproximately constant for all data rates while minimizing theprobability of false detection. A summary of the preamble as a functionof data rates is shown in Table 3. Note that the preamble comprises 3.1percent or less of a data packet. TABLE 3 Preamble Parameters PreamblePuncture Duration Data Rate Walsh (Kbps) Symbols PN chips Overhead 38.432 512 1.6% 76.8 16 256 1.6% 153.6 8 128 1.6% 307.2 4 64 1.6% 614.4 3 482.3% 1228.8 4 64 3.1% 2457.6 2 32 3.1%

[0122] VIII. Forward Link Traffic Frame Format

[0123] In the exemplary embodiment, each data packet is formatted by theadditions of frame check bits, code tail bits, and other control fields.In this specification, an octet is defined as 8 information bits and adata unit is a single octet and comprises 8 information bits.

[0124] In the exemplary embodiment, the forward link supports two datapacket formats which are illustrated in FIGS. 4E and 4F. Packet format410 comprises five fields and packet format 430 comprises nine fields.Packet format 410 is used when the data packet to be transmitted tomobile station 6 contains enough data to completely fill all availableoctets in DATA field 418. If the amount of data to be transmitted isless than the available octets in DATA field 418, packet format 430 isused. The unused octets are padded with all zeros and designated asPADDING field 446.

[0125] In the exemplary embodiment, frame check sequence (FCS) fields412 and 432 contain the CRC parity bits which are generated by CRC gene112 (see FIG. 3A) in accordance with a predetermined generatorpolynomial. In the exemplary embodiment, the CRC polynomial isg(x)=x¹⁶+x¹²+x⁵+1, although other polynomials can be used and are withinthe scope of the present invention. In the exemplary embodiment, the CRCbits are calculated over the FMT, SEQ, LEN, DATA, and PADDING fields.This provides error detection over all bits, except the code tail bitsin TAIL fields 420 and 448, transmitted over the traffic channel on theforward link. In the alternative embodiment, the CRC bits are calculatedonly over the DATA field. In the exemplary embodiment, FCS fields 412and 432 contain 16 CRC parity bits, although other CRC generatorsproviding different number of parity bits can be used and are within thescope of the present invention. Although FCS fields 412 and 432 of thepresent invention has been described in the context of CRC parity bits,other frame check sequences can be used and are within the scope of thepresent invention. For example, a check sum can be calculated for thepacket and provided in the FCS field.

[0126] In the exemplary embodiment, frame format (FMT) fields 414 and434 contain one control bit which indicates whether the data framecontains only data octets (packet format 410) or data and padding octetsand zero or more messages (packet format 430). In the exemplaryembodiment, a low value for FMT field 414 corresponds to packet format410. Alternatively, a high value for FMT field 434 corresponds to packetformat 430.

[0127] Sequence number (SEQ) fields 416 and 442 identify the first datain data fields 418 and 444, respectively. The sequence number allowsdata to be transmitted out of sequence to mobile station 6, e.g. forretransmission of packets which have been received in error. Theassignment of the sequence number at the data unit level eliminates theneed for frame fragmentation protocol for retransmission. The sequencenumber also allows mobile station 6 to detect duplicate data units. Uponreceipt of the FMT, SEQ, and LEN fields, mobile station 6 is able todetermine which data units have been received at each time slot withoutthe use of special signaling messages.

[0128] The number of bits assigned to represent the sequence number isdependent on the maximum number of data units which can be transmittedin one time slot and the worse case data retransmission delays. In theexemplary embodiment, each data unit is identified by a 24-bit sequencenumber. At the 2.4576 Mbps data rate, the maximum number of data unitswhich can be transmitted at each slot is approximately 256. Eight bitsare required to identify each of the data units. Furthermore, it can becalculated that the worse case data retransmission delays are less than500 msec. The retransmission delays include the time necessary for aNACK message by mobile station 6, retransmission of the data, and thenumber of retransmission attempts caused by the worse case burst errorruns. Therefore, 24 bits allows mobile station 6 to properly identifythe data units being received without ambiguity. The number of bits inSEQ fields 416 and 442 can be increased or decreased, depending on thesize of DATA field 418 and the retransmission delays. The use ofdifferent number of bits fOR SEQ fields 416 and 442 are within the scopeof the present invention.

[0129] When base station 4 has less data to transmit to mobile station 6than the space available in DATA field 418, packet format 430 is used.Packet format 430 allows base station 4 to transmit any number of dataunits, up to the maximum number of available data units, to mobilestation 6. In the exemplary embodiment, a high value for FMT field 434indicates that base station 4 is transmitting packet format 430. Withinpacket format 430, LEN field 440 contains the value of the number ofdata units being transmitted in that packet. In the exemplaryembodiment, LEN field 440 is 8 bits in length since DATA field 444 canrange from 0 to 255 octets.

[0130] DATA fields 418 and 444 contain the data to be transmitted tomobile station 6. In the exemplary embodiment, for packet format 410,each data packet comprises 1024 bits of which 992 are data bits.However, variable length data packets can be used to increase the numberof information bits and are within the scope of the present invention.For packet format 430, the size of DATA field 444 is determined by LENfield 440.

[0131] In the exemplary embodiment, packet format 430 can be used totransmit zero or more signaling messages. Signaling length (SIG LEN)field 436 contains the length of the subsequent signaling messages, inoctets. In the exemplary embodiment, SIG LEN field 436 is 8 bits inlength. SIGNALING field 438 contains the signaling messages. In theexemplary embodiment, each signaling message comprises a messageidentification (MESSAGE ID) field, a message length (LEN) field, and amessage payload, as described below.

[0132] PADDING field 446 contains padding octets which, in the exemplaryembodiment, are set to 0x00 (hex). PADDING field 446 is used becausebase station 4 may have fewer data octets to transmit to mobile station6 than the number of octets available in DATA field 418. When thisoccurs, PADDING field 446 contains enough padding octets to fill theunused data field. PADDING field 446 is variable length and depends onthe length of DATA field 444.

[0133] The last field of packet formats 410 and 430 is TAIL fields 420and 448, respectively. TAIL fields 420 and 448 contain the zero (0x0)code tail bits which are used to force encoder 114 (see FIG. 3A) into aknown state at the end of each data packet. The code tail bits allowencoder 114 to succinctly partition the packet such that only bits fromone packet are used in the encoding process. The code tail bits alsoallow the decoder within mobile station 6 to determine the packetboundaries during the decoding process. The number of bits in TAILfields 420 and 448 depends on the design of encoder 114. In theexemplary embodiment, TAIL fields 420 and 448 are long enough to forceencoder 114 to a known state.

[0134] The two packet formats described above are exemplary formatswhich can be used to facilitate transmission of data and signalingmessages. Various other packet formats can be create to meet the needsof a particular communication system. Also, a communication system canbe designed to accommodate more than the two packet formats describedabove.

[0135] IX. Forward Link Control Channel Frame

[0136] In the present invention, the traffic channel is also used totransmit messages from base station 4 to mobile stations 6. The types ofmessages transmitted include: (1) handoff direction messages, (2) pagingmessages (e.g. to page a specific mobile station 6 that there is data inthe queue for that mobile station 6), (3) short data packets for aspecific mobile station 6, and (4) ACK or NACK messages for the reverselink data transmissions (to be described later herein). Other types ofmessages can also be transmitted on the control channel and are withinthe scope of the present invention. Upon completion of the call set upstage, mobile station 6 monitors the control channel for paging messagesand begins transmission of the reverse link pilot signal.

[0137] In the exemplary embodiment, the control channel is timemultiplexed with the traffic data on the traffic channel, as shown inFIG. 4A. Mobile stations 6 identify the control message by detecting apreamble which as been covered with a predetermined PN code. In theexemplary embodiment, the control messages are transmitted at a fixedrate which is determined by mobile station 6 during acquisition. In thepreferred embodiment, the data rate of the control channel is 76.8 Kbps.

[0138] The control channel transmits messages in control channelcapsules. The diagram of an exemplary control channel capsule is shownin FIG. 4G. In the exemplary embodiment, each capsule comprises preamble462, the control payload, and CRC parity bits 474. The control payloadcomprises one or more messages and, if necessary, padding bits 472. Eachmessage comprises message identifier (MSG ID) 464, message length (LEN)466, optional address (ADDR) 468 (e.g., if the message is directed to aspecific mobile station 6), and message payload 470. In the exemplaryembodiment, the messages are aligned to octet boundaries. The exemplarycontrol channel capsule illustrated in FIG. 4G comprises two broadcastmessages intended for all mobile stations 6 and one message directed ata specific mobile station 6. MSG ID field 464 determines whether or notthe message requires an address field (e.g. whether it is a broadcast ora specific message).

[0139] X. Forward Link Pilot Channel

[0140] In the present invention, a forward link pilot channel provides apilot signal which is used by mobile stations 6 for initial acquisition,phase recovery, timing recovery, and ratio combining. These uses aresimilar to that of the CDMA communication systems which conform to IS-95standard. In the exemplary embodiment, the pilot signal is also used bymobile stations 6 to perform the C/I measurement.

[0141] The exemplary block diagram of the forward link pilot channel ofthe present invention is shown in FIG. 3A. The pilot data comprises asequence of all zeros (or all ones) which is provided to multiplier 156.Multiplier 156 covers the pilot data with Walsh code W₀. Since Walshcode W₀ is a sequence of all zeros, the output of multiplier 156 is thepilot data. The pilot data is time multiplexed by MUX 162 and providedto the I Walsh channel which is spread by the short PN_(I) code withincomplex multiplier 214 (see FIG. 3B). In the exemplary embodiment, thepilot data is not spread with the long PN code, which is gated offduring the pilot burst by MUX 234, to allow reception by all mobilestations 6. The pilot signal is thus an unmodulated BPSK signal.

[0142] A diagram illustrating the pilot signal is shown in FIG. 4B. Inthe exemplary embodiment, each time slot comprises two pilot bursts 306a and 306 b which occur at the end of the first and third quarters ofthe time slot. In the exemplary embodiment, each pilot burst 306 is 64chips in duration (Tp=64 chips). In the absence of traffic data orcontrol channel data, base station 4 only transmits the pilot and powercontrol bursts, resulting in a discontinuous waveform bursting at theperiodic rate of 1200 Hz. The pilot modulation parameters are tabulatedin Table 4.

[0143] XI. Reverse Link Power Control

[0144] In the present invention, the forward link power control channelis used to send the power control command which is used to control thetransmit power of the reverse link transmission from remote station 6.On the reverse link, each transmitting mobile station 6 acts as a sourceof interference to all other mobile stations 6 in the network. Tominimize interference on the reverse link and maximize capacity, thetransmit power of each mobile station 6 is controlled by two powercontrol loops. In the exemplary embodiment, the power control loops aresimilar to that of the CDMA system disclosed in detail in U.S. Pat. No.5,056,109, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSIONPOWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM”, assigned to theassignee of the present invention and incorporated by reference herein.Other power control mechanism can also be contemplated and are withinthe scope of the present invention.

[0145] The first power control loop adjusts the transmit power of mobilestation 6 such that the reverse link signal quality is maintained at aset level. The signal quality is measured as theenergy-per-bit-to-noise-plus-interference ratio E_(b)/I_(o) of thereverse link signal received at base station 4. The set level isreferred to as the E_(b)/I_(o) set point. The second power control loopadjusts the set point such that the desired level of performance, asmeasured by the frame-error-rate (FER), is maintained. Power control iscritical on the reverse link because the transmit power of each mobilestation 6 is an interference to other mobile stations 6 in thecommunication system. Minimizing the reverse link transmit power reducesthe interference and increases the reverse link capacity.

[0146] Within the first power control loop, the E_(b)/I_(o) of thereverse link signal is measured at base station 4. Base station 4 thencompares the measured E_(b)/I_(o) with the set point. If the measuredE_(b)/I_(o) is greater than the set point, base station 4 transmits apower control message to mobile station 6 to decrease the transmitpower. Alternatively, if the measured E_(b)/I_(o) is below the setpoint, base station 4 transmits a power control message to mobilestation 6 to increase the transmit power. In the exemplary embodiment,the power control message is implemented with one power control bit. Inthe exemplary embodiment, a high value for the power control bitcommands mobile station 6 to increase its transmit power and a low valuecommands mobile station 6 to decrease its transmit power.

[0147] In the present invention, the power control bits for all mobilestations 6 in communication with each base station 4 are transmitted onthe power control channel. In the exemplary embodiment, the powercontrol channel comprises up to 32 orthogonal channels which are spreadwith the 16-bit Walsh covers. Each Walsh channel transmits one reversepower control (RPC) bit or one FAC bit at periodic intervals. Eachactive mobile station 6 is assigned an RPC index which defines the Walshcover and QPSK modulation phase (e.g. inphase or quadrature) fortransmission of the RPC bit stream destined for that mobile station 6.In the exemplary embodiment, the RPC index of 0 is reserved for the FACbit.

[0148] The exemplary block diagram of the power control channel is shownin FIG. 3A. The RPC bits are provided to symbol repeater 150 whichrepeats each RPC bit a predetermined number of times. The repeated RPCbits are provided to Walsh cover element 152 which covers the bits withthe Walsh covers corresponding to the RPC indices. The covered bits areprovided to gain element 154 which scales the bits prior to modulationso as to maintain a constant total transmit power. In the exemplaryembodiment, the gains of the RPC Walsh channels are normalized so thatthe total RPC channel power is equal to the total available transmitpower. The gains of the Walsh channels can be varied as a function oftime for efficient utilization of the total base station transmit powerwhile maintaining reliable RPC transmission to all active mobilestations 6. In the exemplary embodiment, the Walsh channel gains ofinactive mobile stations 6 are set to zero. Automatic power control ofthe RPC Walsh channels is possible using estimates of the forward linkquality measurement from the corresponding DRC channel from mobilestations 6. The scaled RPC bits from gain element 154 are provided toMUX 162.

[0149] In the exemplary embodiment, the RPC indices of 0 through 15 areassigned to Walsh covers W₀ through W₁₅, respectively, and aretransmitted around the first pilot burst within a slot (RPC bursts 304in FIG. 4C). The RPC indices of 16 through 31 are assigned to Walshcovers W₀ through W₁₅, respectively, and are transmitted around thesecond pilot burst within a slot (RPC bursts 308 in FIG. 4C). In theexemplary embodiment, the RPC bits are BPSK modulated with the evenWalsh covers (e.g., W₀, W₂, W₄, etc.) modulated on the inphase signaland the odd Walsh covers (e.g., W₁, W₃, W₅, etc.) modulated on thequadrature signal. To reduce the peak-to-average envelope, it ispreferable to balance the inphase and quadrature power. Furthermore, tominimize cross-talk due to demodulator phase estimate error, it ispreferable to assign orthogonal covers to the inphase and quadraturesignals.

[0150] In the exemplary embodiment, up to 31 RPC bits can be transmittedon 31 RPC Walsh channels in each time slot. In the exemplary embodiment,15 RPC bits are transmitted on the first half slot and 16 RPC bits aretransmitted on the second half slot. The RPC bits are combined bysummers 212 (see FIG. 3B) and the composite waveform of the powercontrol channel is as shown is in FIG. 4C.

[0151] A timing diagram of the power control channel is illustrated inFIG. 4B. In the exemplary embodiment, the RPC bit rate is 600 bps, orone RPC bit per time slot. Each RPC bit is time multiplexed andtransmitted over two RPC bursts (e.g., RPC bursts 304 a and 304 b), asshown in FIGS. 4B and 4C. In the exemplary embodiment, each RPC burst is32 PN chips (or 2 Walsh symbols) in width (Tpc=32 chips) and the totalwidth of each RPC bit is 64 PN chips (or 4 Walsh symbols). Other RPC bitrates can be obtained by changing the number of symbol repetition. Forexample, an RPC bit rate of 1200 bps (to support up to 63 mobilestations 6 simultaneously or to increase the power control rate) can beobtained by transmitting the first set of 31 RPC bits on RPC bursts 304a and 304 b and the second set of 32 RPC bits on RPC bursts 308 a and308 b. In this case, all Walsh covers are used in the inphase andquadrature signals. The modulation parameters for the RPC bits aresummarized in Table 4. TABLE 4 Pilot and Power Control ModulationParameters Parameter RPC FAC Pilot Units Rate 600 75 1200 Hz Modulationformat QPSK QPSK BPSK Duration of control bit 64 1024 64 PN chips Repeat4 64 4 symbols

[0152] The power control channel has a bursty nature since the number ofmobile stations 6 in communication with each base station 4 can be lessthan the number of available RPC Walsh channels. In this situation, someRPC Walsh channels are set to zero by proper adjustment of the gains ofgain element 154.

[0153] In the exemplary embodiment, the RPC bits are transmitted tomobile stations 6 without coding or interleaving to minimize processingdelays. Furthermore, the erroneous reception of the power control bit isnot detrimental to the data communication system of the presentinvention since the error can be corrected in the next time slot by thepower control loop.

[0154] In the present invention, mobile stations 6 can be in softhandoff with multiple base stations 4 on the reverse link. The methodand apparatus for the reverse link power control for mobile station 6 insoft handoff is disclosed in the aforementioned U.S. Pat. No. 5,056,109.Mobile station 6 in soft handoff monitors the RPC Walsh channel for eachbase station 4 in the active set and combines the RPC bits in accordancewith the method disclosed in the aforementioned U.S. Pat. No. 5,056,109.In the first embodiment, mobile station 6 performs the logic OR of thedown power commands. Mobile station 6 decreases the transmit power ifany one of the received RPC bits commands mobile station 6 to decreasethe transmit power. In the second embodiment, mobile station 6 in softhandoff can combine the soft decisions of the RPC bits before making ahard decision. Other embodiments for processing the received RPC bitscan be contemplated and are within the scope of the present invention.

[0155] In the present invention, the FAC bit indicates to mobilestations 6 whether or not the traffic channel of the associated pilotchannel will be transmitting on the next half frame. The use of the FACbit improves the C/I estimate by mobile stations 6, and hence the datarate request, by broadcasting the knowledge of the interferenceactivity. In the exemplary embodiment, the FAC bit only changes at halfframe boundaries and is repeated for eight successive time slots,resulting in a bit rate of 75 bps. The parameters for the FAC bit islisted in Table 4.

[0156] Using the FAC bit, mobile stations 6 can compute the C/Imeasurement as follows: $\begin{matrix}{{\left( \frac{C}{I} \right)_{i} = \frac{C_{i}}{I - {\sum\limits_{j \neq i}{\left( {1 - \alpha_{j}} \right)C_{j}}}}},} & (3)\end{matrix}$

[0157] where (C/I)_(i) is the C/I measurement of the i^(th) forward linksignal, C_(i) is the total received power of the i^(th) forward linksignal, C_(j) is the received power of the j^(th) forward link signal, Iis the total interference if all base stations 4 are transmitting, α_(j)is the FAC bit of the j^(th) forward link signal and can be 0 or 1depending on the FAC bit.

[0158] XII. Reverse Link Data Transmission

[0159] In the present invention, the reverse link supports variable ratedata transmission. The variable rate provides flexibility and allowsmobile stations 6 to transmit at one of several data rates, depending onthe amount of data to be transmitted to base station 4. In the exemplaryembodiment, mobile station 6 can transmit data at the lowest data rateat any time. In the exemplary embodiment, data transmission at higherdata rates requires a grant by base station 4. This implementationminimizes the reverse link transmission delay while providing efficientutilization of the reverse link resource.

[0160] An exemplary illustration of the flow diagram of the reverse linkdata transmission of the present invention is shown in FIG. 8.Initially, at slot n, mobile station 6 performs an access probe, asdescribed in the aforementioned U.S. Pat. No. 5,289,527, to establishthe lowest rate data channel on the reverse link at block 802. In thesame slot n, base station 4 demodulates the access probe and receivesthe access message at block 804. Base station 4 grants the request forthe data channel and, at slot n+2, transmits the grant and the assignedRPC index on the control channel, at block 806. At slot n+2, mobilestation 6 receives the grant and is power controlled by base station 4,at block 808. Beginning at slot n+3, mobile station 6 startstransmitting the pilot signal and has immediate access to the lowestrate data channel on the reverse link.

[0161] If mobile station 6 has traffic data and requires a high ratedata channel, mobile station 6 can initiate the request at block 810. Atslot n+3, base station 4 receives the high speed data request, at block812. At slot n+5, base station 4 transmits the grant on the controlchannel, at block 814. At slot n+5, mobile station 6 receives the grantat block 816 and begins high speed data transmission on the reverse linkstarting at slot n+6, at block 818.

[0162] XIII. Reverse Link Architecture

[0163] In the data communication system of the present invention, thereverse link transmission differs from the forward link transmission inseveral ways. On the forward link, data transmission typically occursfrom one base station 4 to one mobile station 6. However, on the reverselink, each base station 4 can concurrently receive data transmissionsfrom multiple mobile stations 6. In the exemplary embodiment, eachmobile station 6 can transmit at one of several data rates depending onthe amount of data to be transmitted to base station 4. This systemdesign reflects the asymmetric characteristic of data communication.

[0164] In the exemplary embodiment, the time base unit on the reverselink is identical to the time base unit on the forward link. In theexemplary embodiment, the forward link and reverse link datatransmissions occur over time slots which are 1.667 msec in duration.However, since data transmission on the reverse link typically occurs ata lower data rate, a longer time base unit can be used to improveefficiency.

[0165] In the exemplary embodiment, the reverse link supports twochannels: the pilot/DRC channel and the data channel. The function andimplementation of each of these channel are described below. Thepilot/DRC channel is used to transmit the pilot signal and the DRCmessages and the data channel is used to transmit traffic data.

[0166] A diagram of the exemplary reverse link frame structure of thepresent invention is illustrated in FIG. 7A. In the exemplaryembodiment, the reverse link frame structure is similar to the forwardlink frame structure shown in FIG. 4A. However, on the reverse link, thepilot/DRC data and traffic data are transmitted concurrently on theinphase and quadrature channels.

[0167] In the exemplary embodiment, mobile station 6 transmits a DRCmessage on the pilot/DRC channel at each time slot whenever mobilestation 6 is receiving high speed data transmission. Alternatively, whenmobile station 6 is not receiving high speed data transmission, theentire slot on the pilot/DRC channel comprises the pilot signal. Thepilot signal is used by the receiving base station 4 for a number offunctions: as an aid to initial acquisition, as a phase reference forthe pilot/DRC and the data channels, and as the source for the closedloop reverse link power control.

[0168] In the exemplary embodiment, the bandwidth of the reverse link isselected to be 1.2288 MHz. This bandwidth selection allows the use ofexisting hardware designed for a CDMA system which conforms to the IS-95standard. However, other bandwidths can be utilized to increase capacityand/or to conform to system requirements. In the exemplary embodiment,the same long PN code and short PN_(I) and PN_(Q) codes as thosespecified by the IS-95 standard are used to spread the reverse linksignal. In the exemplary embodiment, the reverse link channels aretransmitted using QPSK modulation. Alternatively, OQPSK modulation canbe used to minimize the peak-to-average amplitude variation of themodulated signal which can result in improved performance. The use ofdifferent system bandwidth, PN codes, and modulation schemes can becontemplated and are within the scope of the present invention.

[0169] In the exemplary embodiment, the transmit power of the reverselink transmissions on the pilot/DRC channel and the data channel arecontrolled such that the E_(b)/I_(o) of the reverse link signal, asmeasured at base station 4, is maintained at a predetermined E_(b)/I_(o)set point as discussed in the aforementioned U.S. Pat. No. 5,506,109.The power control is maintained by base stations 4 in communication withthe mobile station 6 and the commands are transmitted as the RPC bits asdiscussed above.

[0170] XIV. Reverse Link Data Channel

[0171] A block diagram of the exemplary reverse link architecture of thepresent invention is shown in FIG. 6. The data is partitioned into datapackets and provided to encoder 612. For each data packet, encoder 612generates the CRC parity bits, inserts the code tail bits, and encodesthe data. In the exemplary embodiment, encoder 612 encodes the packet inaccordance with the encoding format disclosed in the aforementioned U.S.patent application Ser. No. 08/743,688. Other encoding formats can alsobe used and are within the scope of the present invention. The encodedpacket from encoder 612 is provided to block interleaver 614 whichreorders the code symbols in the packet. The interleaved packet isprovided to multiplier 616 which covers the data with the Walsh coverand provides the covered data to gain element 618. Gain element 618scales the data to maintain a constant energy-per-bit E_(b) regardlessof the data rate. The scaled data from gain element 618 is provided tomultipliers 650 b and 650 d which spread the data with the PN_Q and PN_Isequences, respectively. The spread data from multipliers 652 b and 650d are provided to filters 652 b and 652 d, respectively, which filterthe data. The filtered signals from filters 652 a and 652 b are providedto summer 654 a and the filtered signals from filter 652 c and 652 d areprovided to summer 654 b. Summers 654 sum the signals from the datachannel with the signals from the pilot/DRC channel. The outputs ofsummers 654 a and 654 b comprise IOUT and QOUT, respectively, which aremodulated with the inphase sinusoid COS(w_(c)t) and the quadraturesinusoid SIN(w_(c)t), respectively (as in the forward link), and summed(not shown in FIG. 6). In the exemplary embodiment, the traffic data istransmitted on both the inphase and quadrature phase of the sinusoid.

[0172] In the exemplary embodiment, the data is spread with the long PNcode and the short PN codes. The long PN code scrambles the data suchthat the receiving base station 4 is able to identify the transmittingmobile station 6. The short PN code spreads the signal over the systembandwidth. The long PN sequence is generated by long code generator 642and provided to multipliers 646. The short PN_(I) and PN_(Q) sequencesare generated by short code generator 644 and also provided tomultipliers 646 a and 646 b, respectively, which multiply the two setsof sequences to form the PN_I and PN_Q signals, respectively.Timing/control circuit 640 provides the timing reference.

[0173] The exemplary block diagram of the data channel architecture asshown in FIG. 6 is one of numerous architectures which support dataencoding and modulation on the reverse link. For high rate datatransmission, an architecture similar to that of the forward linkutilizing multiple orthogonal channels can also be used. Otherarchitectures, such as the architecture for the reverse link trafficchannel in the CDMA system which conforms to the IS-95 standard, canalso be contemplated and are within the scope of the present invention.

[0174] In the exemplary embodiment, the reverse link data channelsupports four data rates which are tabulated in Table 5. Additional datarates and/or different data rates can be supported and are within thescope of the present invention. In the exemplary embodiment, the packetsize for the reverse link is dependent on the data rate, as shown inTable 5. As described in the aforementioned U.S. patent application Ser.No. 08/743,688, improved decoder performance can be obtained for largerpacket sizes. Thus, different packet sizes than those listed in Table 5can be utilized to improve performance and are within the scope of thepresent invention. In addition, the packet size can be made a parameterwhich is independent of the data rate. TABLE 5 Pilot and Power ControlModulation Parameters Data rates Units Parameter 9.6 19.2 38.4 76.8 KbpsFrame duration 26.66 26.66 13.33 13.33 msec Data packet length 245 491491 1003 bits CRC length 16 16 16 16 bits Code tail bits 5 5 5 5 bitsTotal bits/packet 256 512 512 1024 bits Encoded packet length 1024 20482048 4096 symbols Walsh symbol length 32 16 8 4 chips Request requiredno yes yes yes

[0175] As shown in Table 5, the reverse link supports a plurality ofdata rates. In the exemplary embodiment, the lowest data rate of 9.6Kbps is allocated to each mobile station 6 upon registration with basestation 4. In the exemplary embodiment, mobile stations 6 can transmitdata on the lowest rate data channel at any time slot without having torequest permission from base station 4. In the exemplary embodiment,data transmission at the higher data rates are granted by the selectedbase station 4 based on a set of system parameters such as the systemloading, fairness, and total throughput. An exemplary schedulingmechanism for high speed data transmission is described in detail in theaforementioned U.S. patent application Ser. No. 08/798,951.

[0176] XV. Reverse Link Pilot/DRC Channel

[0177] The exemplary block diagram of the pilot/DRC channel is shown inFIG. 6. The DRC message is provided to DRC encoder 626 which encodes themessage in accordance with a predetermined coding format. Coding of theDRC message is important since the error probability of the DRC messageneeds to be sufficiently low because incorrect forward link data ratedetermination impacts the system throughput performance. In theexemplary embodiment, DRC encoder 626 is a rate (8,4) CRC block encoderwhich encodes the 3-bit DRC message into an 8-bit code word. The encodedDRC message is provided to multiplier 628 which covers the message withthe Walsh code which uniquely identifies the destination base station 4for which the DRC message is directed. The Walsh code is provided byWalsh generator 624. The covered DRC message is provided to multiplexer(MUX) 630 which multiplexes the message with the pilot data. The DRCmessage and the pilot data are provided to multipliers 650 a and 650 cwhich spread the data with the PN_I and PN_Q signals, respectively.Thus, the pilot and DRC message are transmitted on both the inphase andquadrature phase of the sinusoid.

[0178] In the exemplary embodiment, the DRC message is transmitted tothe selected base station 4. This is achieved by covering the DRCmessage with the Walsh code which identifies the selected base station4. In the exemplary embodiment, the Walsh code is 128 chips in length.The derivation of 128-chip Walsh codes are known in the art. One uniqueWalsh code is assigned to each base station 4 which is in communicationwith mobile station 6. Each base station 4 decovers the signal on theDRC channel with its assigned Walsh code. The selected base station 4 isable to decover the DRC message and transmits data to the requestingmobile station 6 on the forward link in response thereto. Other basestations 4 are able to determine that the requested data rate is notdirected to them because these base stations 4 are assigned differentWalsh codes.

[0179] In the exemplary embodiment, the reverse link short PN codes forall base stations 4 in the data communication system is the same andthere is no offset in the short PN sequences to distinguish differentbase stations 4. The data communication system of the present inventionsupports soft handoff on the reverse link. Using the same short PN codeswith no offset allows multiple base stations 4 to receive the samereverse link transmission from mobile station 6 during a soft handoff.The short PN codes provide spectral spreading but do not allow foridentification of base stations 4.

[0180] In the exemplary embodiment, the DRC message carries therequested data rate by mobile station 6. In the alternative embodiment,the DRC message carries an indication of the forward link quality (e.g.,the C/I information as measured by mobile station 6). Mobile station 6can simultaneously receive the forward link pilot signals from one ormore base stations 4 and performs the C/I measurement on each receivedpilot signal. Mobile station 6 then selects the best base station 4based on a set of parameters which can comprise present and previous C/Imeasurements. The rate control information is formatted into the DRCmessage which can be conveyed to base station 4 in one of severalembodiments.

[0181] In the first embodiment, mobile station 6 transmits a DRC messagebased on the requested data rate. The requested data rate is the highestsupported data rate which yields satisfactory performance at the C/Imeasured by mobile station 6. From the C/I measurement, mobile station 6first calculates the maximum data rate which yields satisfactoryperformance. The maximum data rate is then quantized to one of thesupported data rates and designated as the requested data rate. The datarate index corresponding to the requested data rate is transmitted tothe selected base station 4. An exemplary set of supported data ratesand the corresponding data rate indices are shown in Table 1.

[0182] In the second embodiment, wherein mobile station 6 transmits anindication of the forward link quality to the selected base station 4,mobile station 6 transmits a C/I index which represents the quantizedvalue of the C/I measurement. The C/I measurement can be mapped to atable and associated with a C/I index. Using more bits to represent theC/I index allows a finer quantization of the C/I measurement. Also, themapping can be linear or predistorted. For a linear mapping, eachincrement in the C/I index represents a corresponding increase in theC/I measurement. For example, each step in the C/I index can represent a2.0 dB increase in the C/I measurement. For a predistorted mapping, eachincrement in the C/I index can represent a different increase in the C/Imeasurement. As an example, a predistorted mapping can be used toquantize the C/I measurement to match the cumulative distributionfunction (CDF) curve of the C/I distribution as shown in FIG. 10.

[0183] Other embodiments to convey the rate control information frommobile station 6 to base station 4 can be contemplated and are withinthe scope of the present invention. Furthermore, the use of differentnumber of bits to represent the rate control information is also withinthe scope of the present invention. Throughout much of thespecification, the present invention is described in the context of thefirst embodiment, the use of a DRC message to convey the requested datarate, for simplicity.

[0184] In the exemplary embodiment, the C/I measurement can be performedon the forward link pilot signal in the manner similar to that used inthe CDMA system. A method and apparatus for performing the C/Imeasurement is disclosed in U.S. patent application Ser. No. 08/722,763,entitled “METHOD AND APPARATUS FOR MEASURING LINK QUALITY IN A SPREADSPECTRUM COMMUNICATION SYSTEM”, filed Sep. 27, 1996, assigned to theassignee of the present invention and incorporated by reference herein.In summary, the C/I measurement on the pilot signal can be obtained bydespreading the received signal with the short PN codes. The C/Imeasurement on the pilot signal can contain inaccuracies if the channelcondition changed between the time of the C/I measurement and the timeof actual data transmission. In the present invention, the use of theFAC bit allows mobile stations 6 to take into consideration the forwardlink activity when determining the requested data rate.

[0185] In the alternative embodiment, the C/I measurement can beperformed on the forward link traffic channel. The traffic channelsignal is first despread with the long PN code and the short PN codesand decovered with the Walsh code. The C/I measurement on the signals onthe data channels can be more accurate because a larger percentage ofthe transmitted power is allocated for data transmission. Other methodsto measure the C/I of the received forward link signal by mobile station6 can also be contemplated and are within the scope of the presentinvention.

[0186] In the exemplary embodiment, the DRC message is transmits in thefirst half of the time slot (see FIG. 7A). For an exemplary time slot of1.667 msec, the DRC message comprises the first 1024 chips or 0.83 msecof the time slot. The remaining 1024 chips of time are used by basestation 4 to demodulate and decode the message. Transmission of the DRCmessage in the earlier portion of the time slot allows base station 4 todecode the DRC message within the same time slot and possibly transmitdata at the requested data rate at the immediate successive time slot.The short processing delay allows the communication system of thepresent invention to quickly adopt to changes in the operatingenvironment.

[0187] In the alternative embodiment, the requested data rate isconveyed to base station 4 by the use of an absolute reference and arelative reference. In this embodiment, the absolute referencecomprising the requested data rate is transmitted periodically. Theabsolute reference allows base station 4 to determine the exact datarate requested by mobile station 6. For each time slots betweentransmissions of the absolute references, mobile station 6 transmits arelative reference to base station 4 which indicates whether therequested data rate for the upcoming time slot is higher, lower, or thesame as the requested data rate for the previous time slot.Periodically, mobile station 6 transmits an absolute reference. Periodictransmission of the data rate index allows the requested data rate to beset to a known state and ensures that erroneous receptions of relativereferences do not accumulate. The use of absolute references andrelative references can reduce the transmission rate of the DRC messagesto base station 6. Other protocols to transmit the requested data ratecan also be contemplated and are within the scope of the presentinvention.

[0188] XVI. Reverse Link Access Channel

[0189] The access channel is used by mobile station 6 to transmitmessages to base station 4 during the registration phase. In theexemplary embodiment, the access channel is implemented using a slottedstructure with each slot being accessed at random by mobile station 6.In the exemplary embodiment, the access channel is time multiplexed withthe DRC channel.

[0190] In the exemplary embodiment, the access channel transmitsmessages in access channel capsules. In the exemplary embodiment, theaccess channel frame format is identical to that specified by the IS-95standard, except that the timing is in 26.67 msec frames instead of the20 msec frames specified by IS-95 standard. The diagram of an exemplaryaccess channel capsule is shown in FIG. 7B. In the exemplary embodiment,each access channel capsule 712 comprises preamble 722, one or moremessage capsules 724, and padding bits 726. Each message capsule 724comprises message length (MSG LEN) field 732, message body 734, and CRCparity bits 736.

[0191] XVII. Reverse Link NACK Channel

[0192] In the present invention, mobile station 6 transmits the NACKmessages on the data channel. The NACK message is generated for eachpacket received in error by mobile station 6. In the exemplaryembodiment, the NACK messages can be transmitted using the Blank andBurst signaling data format as disclosed in the aforementioned U.S. Pat.No. 5,504,773.

[0193] Although the present invention has been described in the contextof a NACK protocol, the use of an ACK protocol can be contemplated andare within the scope of the present invention.

[0194] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

We claim:
 1. A method for high speed packet data transmission from atleast one base station to a mobile station comprising the steps of:paging a mobile station of a pending data transmission; measuring C/I offorward link signals from said at least one base station; selecting aselected base station based on a set of parameters; identifying saidselected base station; sending a data request message to said selectedbase station; and transmitting data from said selected base station at adata rate in accordance with said data request message.
 2. The method ofclaim 1 wherein said measuring, selecting, identifying, and sendingsteps are performed at each time slot until said data transmission iscompleted.
 3. The method of claim 1 wherein said measuring step isperformed by taking into account a received value of a forward activitybit.
 4. The method of claim 1 wherein said measuring step is performedon forward link pilot signals from all base stations in an active set ofsaid mobile station.
 5. The method of claim 4 wherein an additional basestation is added to said active set of said mobile station if a transmitpower of said additional base station exceeds a predetermined threshold.6. The method of claim 1 wherein said selecting step is based on C/I ofsaid forward link signals.
 7. The method of claim 1 wherein saidselecting step is based on present and previous C/I of said forward linksignals.
 8. The method of claim 1 wherein said selecting step isperformed in accordance with a predetermined hysterisis.
 9. The methodof claim 8 wherein said predetermined hysterisis is a time basedhysterisis.
 10. The method of claim 8 wherein said predeterminedhysterisis is a level based hysterisis.
 11. The method of claim 1wherein said sending step is performed by covering said data requestmessage with a Walsh code corresponding to said selected base station.12. The method of claim 11 wherein said Walsh code is 128 chips inlength.
 13. The method of claim 1 wherein said data request message isindicative of a requested data rate.
 14. The method of claim 13 whereinsaid requested data rate is one of a plurality of supportable datarates.
 15. The method of claim 14 wherein said supportable data ratesare selected in accordance with a cumulative distribution function ofC/I within a cell within which said mobile station and said selectedbase station reside.
 16. The method of claim 1 wherein said data requestmessage is indicative of a quality of a transmission link.
 17. Themethod of claim 1 wherein said data request message occupies an earlierportion of a time slot.
 18. The method of claim 1 wherein saidtransmitting step is scheduled by a scheduler based on a priority ofsaid mobile station.
 19. The method of claim 1 wherein said transmittingstep is from at most one of said at least one base station at each timeslot.
 20. The method of claim 1 wherein said selected base stationtransmits to one mobile station at each time slot.
 21. The method ofclaim 1 wherein said selected base station transmits at or near amaximum available transmit power for said selected base station.
 22. Themethod of claim 1 wherein said transmitting step is performed usingorthogonal Walsh channels.
 23. The method of claim 22 wherein eachorthogonal Walsh channel has a fixed data rate.
 24. The method of claim1 wherein said transmitting step is performed using quadrature phaseshift keying.
 25. The method of claim 1 wherein said transmitting stepis performed using quadrature amplitude modulation.
 26. The method ofclaim 1 wherein said transmitting step is performed using a directionalbeam.
 27. The method of claim 1 wherein said data is transmitted to saidmobile station in data packets.
 28. The method of claim 27 wherein saiddata packets are of fixed size for all data rates.
 29. The method ofclaim 27 wherein said data packets are transmitted over one or more timeslots.
 30. The method of claim 27 wherein each data packet comprises apreamble.
 31. The method of claim 30 wherein said preamble is spreadwith a long PN code.
 32. The method of claim 30 wherein a length of saidpreamble is based on said data rate.
 33. The method of claim 27 whereineach data packet comprises data units and wherein each data unit isidentified by a sequence number.
 34. The method of claim 33 furthercomprising the step of: transmitting a negative acknowledgment (NACK)messages for data units not received by said mobile station.
 35. Themethod of claim 34 further comprising the step of: retransmitting saiddata units not received by said mobile station in accordance with saidNACK messages.
 36. The method of claim 1 further comprising the step of:sending data to all base stations in an active set of said mobilestation.
 37. The method of claim 36 wherein said selected base stationtransmits based on a predictive determination of remaining data.
 38. Amethod for high speed packet data transmission from at least one basestation to a mobile station in a CDMA communication system comprisingthe steps of: first transmitting a pilot signal from each of said atleast one base station; measuring C/I of said pilot signals from said atleast one base station; selecting a selected base station based on a setof parameters; identifying said selected base station; sending a datarequest message to said selected base station; and second transmittingdata from said selected base station at a data rate in accordance withsaid data request message.
 39. The method of claim 38 wherein saidmeasuring, selecting, identifying, and sending steps are performed ateach time slot until said data transmission is completed.
 40. The methodof claim 38 wherein said sending step is performed by covering said datarequest message with a Walsh code corresponding to said selected basestation.
 41. The method of claim 38 wherein said data request message isindicative of a requested data rate.
 42. The method of claim 38 whereinsaid data request message is indicative of a quality of a transmissionlink.
 43. The method of claim 38 wherein said selected base stationtransmits to one mobile station at each time slot.
 44. The method ofclaim 38 wherein said selected base station transmits at or near amaximum available transmit power for said selected base station.
 45. Themethod of claim 38 wherein data is transmitted to said mobile station indata packets, wherein said data packets are transmitted over one or moretime slots.
 46. The method of claim 45 wherein each data packetcomprises data units and wherein each data unit is identified by asequence number.
 47. The method of claim 46 further comprising the stepof: transmitting a negative acknowledgment (NACK) messages for dataunits not received by said mobile station.
 48. The method of claim 47further comprising the step of: retransmitting said data units notreceived by said mobile station in accordance with said NACK messages.49. An apparatus for high speed packet data transmission from at leastone base station to a mobile station comprising: a transmitter withineach of said at least one base station for transmitting paging messageswithin a forward link signal to said mobile station; a receiver withinsaid one mobile station for receiving said paging messages andperforming C/I measurements of said forward link signals from saidtransmitters within said at least one base station; a controller withineach of said at least one mobile station, said controller connected tosaid receiver for receiving said C/I measurements, said controlleridentifying a selected base station; a transmitter within said mobilestation connected to said controller for transmitting data requestmessages; and wherein said transmitter within said selected base stationtransmits data at a data rate in accordance with said data requestmessage.
 50. The apparatus of claim 49 wherein said receiver performssaid C/I measurements at each time slot; said controller identifyingsaid selected base station at each time slot, and said transmitterwithin said mobile station transmits said data request message at eachtime slot.
 51. The apparatus of claim 49 wherein said receiver performssaid C/I measurements by taking into account a received value of aforward activity bit.
 52. The apparatus of claim 49 wherein saidtransmitter within said mobile station further comprises: a Walsh coverelement for covering said data request message with a Walsh codecorresponding to said selected base station.
 53. The apparatus of claim49 wherein each of said at least one base station further comprises aqueue for storing data.
 54. A method for high speed packet datatransmission from a mobile station to at least one base stationcomprising the steps of: sending a request for high rate transmission atone of a plurality of supportable data rates on a reverse link signal;receiving and granting said request for high rate transmission; sendingsaid grant to said mobile station; and transmitting data at said one ofa plurality of supportable data rates.
 55. The method of claim 54wherein said mobile station transmits data a low data rate without agrant from said at least one base station.
 56. A transmitter for highspeed packet data transmission comprising: an encoder for receiving datapackets and encoding said data packets into encoded packets; a framepuncture element for receiving said encoded packets and puncturing aportion of said encoded packets to provided punctured packets; avariable rate controller connected to said frame puncture element forreceiving said punctured packets and demultiplexing said puncturedpackets into parallel channels; a Walsh cover element connected to saidvariable rate controller for receiving said parallel channels andcovering said parallel channels with Walsh covers to provide orthogonalchannels; and a gain element connected to said Walsh cover element forreceiving said orthogonal channels and scaling said orthogonal channelsto provide scaled channels.
 57. The transmitter of claim 56 wherein saideach of said parallel channels has a fixed data rate.
 58. Thetransmitter of claim 56 further comprising: a multiplexer connected tosaid gain element, said multiplexer multiplexing pilot and power controlbursts with said scaled channels to provide Walsh channels.
 59. Thetransmitter of claim 58 wherein said pilot and power control bursts arelocated at fixed locations within each time slot.
 60. The transmitter ofclaim 58 wherein said pilot and power control bursts are provided at twolocations within each time slot.
 61. The transmitter of claim 56 furthercomprising: a multiplexer connected to said gain element, saidmultiplexer multiplexing a preamble with said scaled channels to provideWalsh channels.
 62. The transmitter of claim 56 further comprising: ascrambler interposed between said frame puncture element and saidvariable rate controller, said scrambler scrambling said puncturedpackets with a scrambling sequence.
 63. The transmitter of claim 56wherein said each of said Walsh covers is 16 bits in length.